I also run into a similar situation when shooting nudes but being a grown man, I have a much tougher time hiding that than a woman probably would. Unless of course, she's a shemale, in which case we're on even ground. Whenever I do a shoot with a beautiful woman or an androgynous young man, my trembling truncheon of terror has to be tucked back. Either that or I have to think of something completely unrelated that doesn't make me aroused. Usually this means thinking of something or someone I hate, like Glenn Beck or The Police (The band, not the law enforcement officials). The fastest way for me to kick myself back into reality is to picture Gabe Newell's face, then I can't get aroused for days.
In any case, having your body respond to the presence of a naked body is perfectly natural. As long as you don't misbehave, it's not an issue. The only person who really will know about it is you, and as long as you keep to yourself and don't openly display your proud girl-boner, the model will leave none the wiser.
I don't like you comparing bisexuality to you being unable to keep your crotch dry seeing a naked body. I'm fucking dikey as hell but I still do fine when a hot chick comes along at life drawing classes or something like that.