Going with girls still in relationships is not the way to go. Trust me, I know. Never had to deal with a big boyfriend though, that just seems to add another problem.
Love is as retarded as
Seriously, you bring love into a situation and it goes from "pretty cool" to "Fuck this, I'mma get me some rope" far too quickly. I've seen it happen a few times now. It's not pretty watching a guy fall in love, get dumped and spend the next year not washing, bathing, changing clothes or talking. Not pretty, but damned funny, in a harsh way.
I guess what I'm trying to say is Good luck.