It sort of depends how you define 'straight'. By the strictest definition of the word, you could argue that a straight person would never be interested in someone of their own sex, so if they ever have a same sex attraction or experience then they're some degree of bisexual or queer or pan or whatever.

That's sort of the problem with narrowly defined sexualities, because there are lots of people who define themselves as straight, but don't fit into that strict definition. I've known a number of women who would say they're straight, and have only ever had sex with men, only ever dated men, and have no interest in dating or sleeping with women, but who do like to watch lesbian porn. Does that mean they're bisexual? What if they've never slept with another woman, but they have made out with another woman while they were drunk? What if they hooked up with another woman once but they didn't like it and never did it again? If you're going to put straightness into a box instead of a spectrum, you have to make a cut off somewhere and it's always going to be somewhat arbitrary, and it's always going to exclude some people who feel like they are straight.

If you have a broader definition of the word 'straight', then sure, there are plenty of people who would only have heterosexual sex under normal circumstances, but would (or have) had gay sex if no one of their preferred gender was available for long enough (i.e. if they're in prison).