In ME1, you'll need to change some of gender markers to romance either Kaidan and Ashley. In ME2, I don't know exactly how but you can use gibbed to allow yourself being in a romance state with someone that's not available to you. But essentially, you're playing a transgendered Shepard (even in ME1 Mark Meer voice some of FemShep's line saying he's a woman).
Except for ME3 Kaidan whose dialogue was written in gender neutral way, (i.e. he never refer Shepard as a woman again), so those who use mods could play with them being in romanced with Kaidan, for everyone else, you will get silent dialogues and maybe some hidden dialogues that was unlocked (like FemShep telling Liara that she cared for Miranda, FemShep flirting with Tali, BroShep flirting with Thane).
But even with mods, sometimes certain dialogues won't get unlocked. For instance, modded Kaidan on Horizon still won't berate BroShep about being a traitor and betraying him.