Bi = 2. Bisexual = you fancy the 2 of them, so both men and women. When you regularly have sexual feelings for both boys and girls, it is called bisexuality. ‘Bi’ means two, or both. Generally you prefer the one slightly to the other.
Besides that, it is quite normal for heterosexuals to have the occasional gay fantasy. But you don’t need to be gay or bisexual for it.
As bisexual you therefore have two possibilities. You can enjoy both the male and female body sexually and have a greater freedom of choice. You might have a relationship with a girl, but at the same time have sex with a guy. Or vice versa.
Many bisexuals choose to go through life as husband and wife. The feelings, fantasies and interest in people of the same sex don’t necessarily disappear because of this choice. It can help if you maintain the ability to discuss these feelings in your relationship. It can be quite a challenge if you have the need for sex with both men and women and wish to maintain a relationship with someone who prefers monogamy. (Monogamy: having sex only with the person with whom you have a relationship).
Be true to yourself and your feelings. You are allowed to think out of the box! Don’t concern yourself with the question of whether you are gay or straight. Be happy that you are fortunate enough to choose not only from 50% but from 100% of mankind.