Someone who imposes a sexual preference upon themselves for some outside reason. A person might have no gender-bias, but decides that it might be easier to have and raise children with someone of the opposite sex. This person imposes a gender restriction upon those that they may get involved with.
'I used to see John date men and women until about a year ago, and now he seems to only date women!'
'oh, didn't you hear? he imposed a gender restriction on who he dates because he decided he wants to find someone he can have a biological child with.'
'whoa, I didn't know people could do something like that!'