A good friend, who is in a serious relationship, has been hitting on me a lot recently.  What should I do?

This is my take on the situation.   Most women do the breaking up, and this isn't because they necessarily want to break up, often it's because the man will be such a jerk, until the woman can no longer put up with his BS, and puts an end to it all.   Usually, there is no real reason for the man to break up - emotionally he may be somewhere else, but he is still getting serviced at 'home'.  He is having it both ways.   And you are the other way.  He is using you.  He is too weak to make the decision, and if he gets you involved, then it's the women making the choice, he is just a bystander picking up what is left.  And remember, you have no idea what he is telling his partner.   l strongly suggest you back off, way off.  Stop seeing him.  Block him, at least for a while.  You need to figure out if he is really an addiction or a candidate as a mate.  He does seem to be lacking boundries, and you may want to really examine his character to determine if he is someone you can respect and love in the widest definition.  Being with him could just put you in the position of his current partner - wondering what he is up to as soon as he is out of your sight.  How will you trust him, when you know he has to be lying about where he is to his current partner?  How can he carry your heart when you know he emotionally invests in women outside the relationship in a primary way?   Good luck.  There is going to be pain here.  I can't see how there can't be.

Well, you seem to be on the right path. Don't hook up with him unless he's out of his relationship for two reasons. 1) Don't ever be the reason for a couple's breakup, it's not worth it and you simply feel horrible for that. 2) If he starts a relationship with you cheating on his actual one, he will certainly do that to you one day. Talk to him about that, just be straight with him (considering it's a him, could be a her)

Instead of second guessing here, why not go for a simple coffee with this person and ask what's going on what they want to do and have a good old fashioned chat about the situation? They might be a golden opportunity for both of you or there might not. Getting the info direct will at least pave the way for the decision that you need to make.

Keep your distance and watch what happens. Either your friend will end her relationship or she won't. If she does, you'll know that any relationship you may have with her was started in an unquestionably "right" way, and if she doesn't, you'll have your answer that she's using you as a means to an end. She may have feelings for you, but they need to be greater than her feelings for her current romantic partner, and stronger than her fears of ending her present relationship, in order for you both to know where she stands.

Assuming that this serious relationship is also at least ostensibly monogamous (iffy, because non-monogamous relationships can be as serious and usually are more serious)... If you win this battle, someone else will win the next one, and you'll be the loser.

I'm not going to make gender assumptions here, because the question seems to have pretty specifically avoided telling us which players are which gender, but I will say I'm writing this as a man. And as a man, I have certainly dragged relationships out a little longer then I should have while making sure I had another option lined up. I guess that's kind of awful, but I can't say that breaks ups that happened within this type of situation have been any worse then ones where I had not already 1/2 moved on. I know the assumption is that this is just a dick move, and I'm not saying it wasn't, but the logic of it is more rooted in insecurity then predatory malice. In particular, there was a situation when I was in my early 20s and had had a girlfriend for more then a year. It wasn't working out anymore, she was not good to me or for me, but I was, frankly, staying with her because not that many women had been into me, and I kind of felt like she was what I deserved even if it wasn't good for me. We had ended up in different cities, and while it was scummy of me to cheat on her, seeing that I could be attractive to other women was also what gave the courage to get out of a bad situation. Maybe this is a reasonable justification, maybe I was just weak, OK I was weak, condemnation, pity, and compassion are all valid judgements to make, in my opinion. If your are an absolutist, as many on Quora are, then this is a violation of Absolute Morality in All Situations and I'm therefore irredeemable scum, but personally, I try and be more compassionate about the whole situation. Point being, I don't think this spells doom necessarily. Maybe you 2 really are right for each other, even if the timing feels wrong. My guess is that it's the 2nd option you listed: they are doing this as a way to break up with that other person, your friend has some insecurity issues, but hell, don't we all. Life is complicated and love shows up in all the wrong places all the time. It's not usually worth it to kick up a lot of drama just so people can get off, but when the stakes are higher then that, the situation gets a lot more complicated and grey. If all this generates a lot of distrust and disgust in you, no need  to get all involved, but if you are really into your friend, can look past all the drama, not lose respect for them and really do want to be with them, I'd still take it slow and make them prove that you are not just a pawn in their relationship, but proceed. If you do hook up with them before they break up with the other person, don't do so more then once. It's one thing to 'give in to the moment' and let something happen that has been building up for a while, to essentially say 'yes I want to be with you, now what' but don't let it turn into a drawn out affair where your friend is actively seeing you and another person for an extended period of time, as that is just your friend playing both you and their sig. All's fair in love and war, but in both cases there is no point in a http://en.wikipedia.org/wiki/Pyrrhic_victory.

Show some "balls" and just don't do it until your friend has broken up. He/she will respect you for your moral fortitude, might even pursue you harder and not consider you an easy conquest and, even if nothing develops between the two of you, you would have provided your friend a fine example. More importantly, you would gain more respect for yourself.

Find someone who is available.  What if this person leaves their relationship for you?  How can you trust that this person wouldn't leave you for someone else?  They are currently cheating.  Doesn't sound like a great start to a lasting relationship to me.

If a good friend been hitting on you, you need to sit down and talk. When a man who is married or in relationship starts hitting on a female friend, it means really distraught state of mind.