There are a number of ways to attempt to interpret this.  Do you have any more context to the dream, or is that all you remember?  Do you remember any colors, emotions, conversations, places, objects in the room?

One way of looking at it would be that your dream is using your boyfriend and your daughters as symbols of something that's actually going on inside of you.  Sex is used as a symbol of unity.  If you think this might make sense, then you'll have to figure out what each of these people represents. 
Your boyfriend could be representing a certain quality that you and he have in common.  What is the first thing that comes to mind when ou think of him?  Aternatively, he could be symbolizing your animus, or your masculine side. 
It is common for women to dream of their younger sisters, daughters, or a young girl.  Often this younger girl is part of their soul representing innocence, vulnerability, or the part that needs both affection and protection.
So you can chew on that and see if it makes sense.

Another way of looking at this is that the characters literally represent the people they look like, but their actions are symbolic, and it is still something going on inside of you.  What I mean is, perhaps your dream means that you are wishing that your boyfriend and your daughter would get along better in real life, or you've started noticing that they are getting along better in real life.  There's a coming to peace with the situation.  I'm of the opinion that sex in dreams seldom means literal sex, but it is usually symbolic of unity, desire, or something along those lines.

Another idea, and judging by your desperate tone, it sounds like you're concerned that this may be the case, is that the dream is telling you of some kind of abuse or inappropriate behavior going on in real life; in this scenario, the dream would be almost completely literal, which is unlikely, in my opinion.  However, perhaps your subconscious has picked up on some clues that your conscious mind hasn't noticed yet, and your dream is warning you that there is trouble between these two people.

You'll have to decide which interpretation seems right to you.  Without more context, and without being you, I could never give you a definitive answer on what the dream means.

I will, if you don't mind, insert a personal comment here.  My parents divorced when I was 12, and both of them found partners shortly afterward.  As a 12/13-year-old, just a child starting to enter adolescence, I was pretty devastated and confused.  Most 13-year-olds are pretty confused anyway, that's just part of being 13!  But to throw a tragic event like divorce onto my life at that tender stage, and then to see both of my parents seeing someone else, my life felt really upside-down.  I don't know you or your daughter at all, of course, but I would just say that it's likely that your daughter is feeling a little overwhelmed.  She needs you more than ever at this tough stage of life, and even though she might try to push you away, deep inside she longs to be with you.  At least that was true of me when I was 13...  My advice (and I apologize for offering it without being asked!!) is to spend lots and lots of time with her, talk with her, be a friend and be real with her, build the relationship as much as you can.

Good luck!