I am attending a bridal shower this week-end for by boyfriends future daughter in-law. We have lived together for 5 years and do not plan on getting married. When they ask at the shower how do you know the bride, who do I answer?

"I'm just here for the cake." Honestly, "My boyfriend is the father of the groom" works fine.

"I used to date her, but it was just for sex."

WHatever you say you'd better find a more direct way than that... people will give you a funny look and wander off. She's marrying your boyfriend's son (however you want to phrase it)

It depends on how you see her. If you two are friends just say you're a friend of the bride. If you consider her or the groom family just say that you are family of the bride/groom.

The groom is my boyfriend's son.

Referring to yourself as her future father-in-law's partner, significant other, girlfriend, or whatever makes you and your partner comfortable works. I would talk to him about it.

Just tell them that she is marrying your boyfriends son.

Quite simply that you know the bride through her father. After all everyone is just trying to make pleasant conversation with people whom they may have never met.

You could either say "I'm the groom's Father's girlfriend" or "well 5 years ago I met the grooms father and we've been dating ever since but don't plan to marry, so I'm not really related to either - only in my heart"