Nature... I'd say one's willingness or ability to ACT on one's biological sexual orientation can be influenced by environment (supportive peers, family, etc) but the desire is either there or it isn't.
I think it's enviroment, because if you say that it's caused by nature then your also passivly saying that our behavior is preset by nature and we can't do anything about it. What i'm trying to say is that people aren't born killers...(I'm not comparing us to killers btw) but society creates them, so like i think its somthing that psychological that's set from a very young age. Either way there is still no control over it thou. Anyway thanks for taking the time to read this.