Well, heterosexuality is defined as an exclusive attraction to members of the opposite sex. When such heterosexuals do have sex with members of the same sex, many people do not say that such people are not heterosexuals. Instead they tinker the definition of heterosexuality with excuses like "There weren't any women around so men will have sex with men." The problem with this so-called situational homosexuality is that one would expect people who are exclusively attracted to members of the opposite sex to refrain from sex with members of their own sex. A genuinely heterosexual man should either be celibate or masturbate* if there are no women around. If he willingly seeks sex from men, he's not exclusively attracted to women and ergo not a heterosexual.
*If you think about it, masturbation is as homo as it gets. Homo is the Greek word for "same," and it doesn't get anymore same than yourself. Maybe I'll break my dryspell and make a video on this.