I don't think anyone has a choice over who they feel attracted to - but people do have a choice in whether they allow those emotions to rule or whether they will engage their brain and make rational decisions. For example, I know that I have a "type", and I do not feel sexually attracted to men who don't fit that type. However, just because someone might be my "type" doesn't make me powerless to resist his charms or unable to determine if we are actually compatible. I am also free to choose to disregard my initial lack of attraction, although in my experience it turned out to be an absolute disaster.
When the number of potential mates is so limited (like within the borg), I think it really makes it hard for people to pair off successfully. Especially because the borg actively discourages open and honest conversations about spirituality, the likelihood of people pairing off who are actually not at all compatible is pretty high. Plus, having to stifle large portions of your natural personality because they are "unchristian" doesn't help. I look back at the person I tried to be while I was in the borg, and I don't even recognize myself.