Men produce semen constantly. If we don't masturbate or have sex the body has a built in mechanism to release semen involuntarily while you sleep. The release is usually triggered by a dream in which you become sexually aroused. Note: You are not in danger of producing too much semen if you don't have a wet dream. If ejaculation does not occur over a period of time, the sperm don't accumulate. They instead are broken down, reabsorbed by the body and recycled.
The fantasy in your dream could be about absolutely anything regardless of how you would normally act in reality. Remember, we cannot be judged or held responsible for our feelings or fantasies. However, you are responsible for what you do in real life. It's perfectly normal for us to dream about warm feelings, deep friendship, love and even erotic feelings toward either sex regardless of our primary sexual orientation. If you have a certain sexual orientation and you dream from time to time about experiencing the opposite, it does not mean your sexual orientation is different that what you've believed it to be. If you dream of nothing but experiencing the opposite sexual orientation, than you might have reason to further search your primary feelings. Otherwise you can be confident that you have been living according to your true sexual orientation. Dreams and fantasies have many functions, one of which is to balance one's life and experiences. If you feel powerless in reality, you might dream of possessing superhuman strength or the ability to fly. If you stress over the size of your manhood, you might dream of being well endowed. If your daytime is spent obsessing over girls, you might dream of an erotic experience with a guy. It is all perfectly normal. Furthermore, some people believe that we are all partly heterosexual and partly homosexual. The deciding factor in your life style is which one are you more predominantly. If you are equally attracted to the same sex and opposite sex than you might consider yourself bisexual. If you are attracted more substantially and consistently to the opposite sex, it's safe to say you're heterosexual. If the reverse is true, it's safe to consider yourself homosexual.