Roly this is a big deal.
It's good for you to see it, and acknowledge YOUR part in it as well as acknowledging the already existing family's part in it.
Maca and I had a family as well. When GG decided to be with me, he gained OUR family, but he also gave up having a family "all his own" so to speak.
Not that he couldn't marry someone and start a family (well could have) but he would always have a SHARED family (now he's fixed but that's another story).
It was REALLY a struggle for his friends/family, even Maca to understand how he could make that choice. But for him (and my sister too) they consider these kids to be their own-even though the children aren't biologically theirs (the little one is bio-child of gg's, but none are my sisters). The kids as well consider all four of us parents...
So there are multiple ways for it to be worked out-but you really have to find the right fit for you (and the people you are with).