Well, uhm, that's where the differences between fundamental (what should be with a perfectly neutral society and upbringing and a perfect sampling of people to meet) and declared (what a person says they'll see themself as) sexual orientation.
Basically, let's say you're 90 % into women and 10 % into men (this you being a general you, not anyone in particular). What are the chances you'll say you're into men? Nah, at those numbers, chances are that you're only into women in all practical aspects. Also, who knows, perhaps a woman with slightly masculine traits would perfectly fit you.
What I'm trying to say might be more easily worded as "most heterosexuals have a capability to enjoy homosexual behaviours, and vice versa". That doesn't mean they'll admit it, and that doesn't mean it'll be strong enough for them to even think about it. And it doesn't mean they should change their "official" sexual orientation. Heck, if it takes the most favourable conditions in the world for you to enjoy a homosexual relationship, in practical terms, you're simply hetero.
And all this is not even taking into account feelings, which complicate matters by a fair bit. Some people, for instance, are asexual at varying levels. Others might be totally turned on by men only, but constantly fall in love with and want to marry women.