Yeah, this is the tricky part. IO think something immediately post TF is the best time for it and just a splash screen with various options for the player. Trying to make it too immersive would just over complicate it
IMHO it is interesting how it was done in a QSP-Player game named Clean Slate (which also is here on F95 Zone).
In it you aren't actually transferred into a girls body, but into a more girlish (but still male) and your body may gradually change in to a full girls one (with you growing brfeasts and your penis shrinking and in the ned becoming turned into a Vagina) ... and well, with sexual preferences, it works like some additional attribute.
I.e., at the beginning you are practically only attracted to women ... but when you are very horny and/or very drunk, you may actually, very reluctantly give in to sexual encounters to men.
But every time you have sexual encounters with men, your sexual preference attribute shifts a little bit more towards bissexual, till at one time kit has shifted enough to actually become bisexual and therefore you have no shy anymore about flirting with men and having sex with them.
If you have more sexual encounters with men, than with women, you may even finally become attracted to men only and disregard women as sexual targets.
So, in a way, in that game you are able to choose your sexual preference (by forcing sexual encounters with men, or staying with having sexual encounters primarily with women) but it is very well integrated into the story telling of the game.
Maybe this would be a way for Girl Life as well.
(likewise in that game, feminity is an attribute ... whether you see yourself more as a male (which is the case in the beginning) or start to see yourself more as a female and develop female behavior)