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.
But won't the model notice that you're aroused? For women, erect nipples, obviously she can't see what is happening down there and for men something going on in the trouser department... Isn't it a bit unprofessional?
try wearing a thicker bra, sweetie. or a scarf laying right. if you hide it it's fine and don't make a big deal about covering it up but don't... show it. i don't know. it's likely normal. i'm not particulary horny, but if you don't want people to see, thicker bra, sleeveless jacket, etc.
Well, being horny doesn't mean you have to be stiff. Nipples showing are a matter of what's covering them, and they tend to show for a greater variety of reasons anyway. Penises take more to get very erect, since they have to pump a more substantial amount of blood into them, so depending on one's metabolism simply being standing and not concentrating all your thoughts on the sexiness is generally sufficient to keep it down, though being healthier/younger might make that less true; I can't say it's ever been an issue for me.