No they're not. Straight is the default for human life through genetics. Without reproduction we would not be here hence evolution. I agree your mind may be altered through possible environments and the like, but sexuality isnt a choice you make.
Lol, assuming attraction and reproduction share the same means of conducting a sexual decision is ignorant at best. Please don't use the logic:
Humans know they need to reproduce to survive, humans must be straight to reproduce= A person understands they need to reproduce to survive, therefor he/she must be straight.
It is flawed.
Reproduction is a physical and survival trait, based off of evolution and our understanding that without this we would die out. Homosexuals even understand this. Saying this causes straight to be humans default is ridiculous, since reproduction as I said is built into humans as a physical liability, not emotional (where the subject of attraction lies).
Sexuality is based on attraction and your own innate desires. It is a choice you make unconsciously based on how you react to your own feelings, surroundings, and many other variables.
Please don't twist my words by saying your mind can be "altered" it is not altered, it is how it always was and will be. The choice of sexuality is "influenced" by all these variables.
You can change your sexuality, but you must actually change and choose to be gay/straight/bi/bestiality/etc completely. So it is possible but since this unconscious decision was made without forcing it before, it would have to be made unconsciously as well, meaning your brain must actually change its attraction from females to males.