There are actually "settings" in the game that help to determine their autonomous gender of romantic preference. Most sims start out as neutral (having no automatic preference), but several sims that do have a preference. But all this really means is that these sims are more likely to initiate romantic interactions with sims of that particular gender at the start of the game. Once the town goes live, story progression takes over, and these preferences can change depending on the romantic interactions they are involved in.
For example: Gobias Koffi's preference is weighted towards males. This means that he is more likely to flirt with men. However, if he develops a high relationship with a female sim and she flirts with him, his preference will take a step back towards neutral. If he continues to be the subject of romantic interactions from female sims, his preference will eventually change. Also, just because his preference is males to start off with doesn't mean that he won't flirt with a female sim he has a high relationship with.
In my games, Gobias ends up in a relationship with a male only slightly more often than he does with a female. But a majority of the time, Gobias lives out his entire life without having any relationship at all (that I am aware of). Madison Van Watson also seems to have a preference for same sex relationships. She has been involved with Tori Kimura and Ayesha Ansari, but more often than not she ends up in a relationship with Monika Morris...leading me to believe that Monika may be weighted slightly towards females, too (however, it may be due to nothing more than their BFF relationship at the start). Also, Jett Atkins in Bridgeport has ended up in a romantic relationship with a male he went to high school with in all 3 games that I've played there. Moe Pesce and Dude LaMer ended up in a relationship once in Barnacle Bay, but only once.
It's also worth noting that it appears that sims you create can end up with a same gender preference, too. A sim I created not too long ago constantly got wishes to kiss her female coworkers for the first time (and never got wishes to kiss males), despite the fact that there was an even split between the two genders.