There is no sexuality spectrum. And, contrary to what doctor house said, it isn't that complex. Sexuality hinges on arousal. Stimulus -> arousal determines our sexuality.
Gay people get aroused from stimulus relating to the same sex, straight people the opposite sex, and bisexuals both sexes. It's as simple as dopaminergic pathways in the brain activating in response to stimulus. These are not learned traits either. They are genetic and developmental.
Twins can have different sexualities; however, that does not suggest that sexuality is learned. Genetics is a blue print, but it isn't fool proof. Other things influence development. Autism can occur in 1 twin, but not the other. But that does not suggest that autism is learned. Development can just "go wrong" so to speak.
Moreover, you can't be a little gay or a little straight. These pathways either activate or not. Mind you, they will activate less in response to a weaker stimulus (eg a less attractive person... 7/10) and more to a stronger stimulus (eg a 10/10). If they do activate even a little, you will get aroused, an erection, etc.
If a man gets aroused by another man, he is either bisexual or gay, both of which are, for lack of a better term, neurological defects. There is no spectrum or continuity. It's rather black and white.
- - - Updated - - -
Our sexuality is not learned. It is evolved. It isn't evolutionarily advantageous to be attracted to goats because you wouldn't have many offspring if you spent all of your time screwing goats.
Likewise, it isn't advantageous to be attracted to the same sex. However, it also isn't advantageous to have diabetes, ALS, autism, cerebral palsy, etc. As I said, it's a defect.