Bk1 is the only one where the routes are basically the same. In bk1 the only difference is rougher dialogue, a slap scene and 1 or 2 other scene differences. I think the blowjob scene changes significantly, I forget. I would fully recommend playing both routes in bk2, 3 and 4 (especially 4). Bk4s routes are like entirely different books, its really well done.
Edit Actually yeah you leave in 2 out of 4 of th endings deep inside Zukos sister. I like to imagine he was looking at the infinite online lewd sites MC must have had tabs of open on his computer when Zuko swaps back at that moment. (I forget if morality adds ending differences in love route so may be more)added perk, imagine their face when you leave their bodies and they're faced by whatever consequences you left them to, or they somehow (maybe the snake..? hrhrhr) get flashbacks into the time you've been screwing around using their bodies.