From my perception, you can have two romances active in Act 1. If those slots are filled, nobody will approach you anymore - or in case of Lae'zel she'll tell you that too many people have nibbled at you already and she doesn't like leftovers. In Act 2 you have to decide, which one you'd like to continue with. So if you, say, started romancing Gale and Shart in Act 1, Wyll won't approach you in Act 2.
Yeah, so if you're a one-lover, be prepared to "fight back". It's even funny though, such a "hidden quest" for the player's discernment, because some of the companions' courtships are somewhat veiled, and you have to be a bit "paranoid" not to get into anything even remotely resembling cheating
I was even afraid to hug Karlach in the scene when she regains the ability to touch other people, although I wanted to do it in a friendly way.
