You may think it unnecessary for me to describe the “natural,” assumed-unless-otherwise-stated, default sexual type to which most people supposedly belong.
In polls, the vast majority of women and men identify themselves as straight, in part because the other options are so limited. Though the term presumably refers only to sexual attraction, you may feel that it also captures your sense of masculinity (if you are a man) or femininity (if you are a woman) and your sense of being sexually and socially normal. Even if you are sexually aroused by, or have had sex with, someone of the same sex, you may feel that identifying as straight better captures your romantic feelings and relationship goals. I encourage you to take the Flexuality Test and consider the other categories, since many straight men and women are probably more accurately classifiable as heteroflexible.
If you are genuinely straight, you are attracted only to members of the opposite sex and cannot imagine yourself in a romantic relationship with anyone else. Since reaching puberty, your fantasies and aspirations have always involved the other sex. If you have had a homosexual experience, you found it uninspiring, if not unpleasant. You may feel that heterosexuality is the natural state and that homosexual feelings are only experienced by gays and lesbians. You may find it hard to even entertain the possibility that you could have such feelings yourself.
On the other hand, you may be open-minded to the idea of sexual flexibility and have close friends of both genders and different sexualities while experiencing no same-sex desires yourself. Some people may just be born that way.