I couldn't find a recent report regarding this but I did find a post about my exact same issue that was posted quite a while ago.
Sims constantly flirting with same sex | EA Forums - 8599684
This exact same thing just happened in my game. It's not the first time it has but it is the first time I realized fairly quickly. I had to get a mod to fix it because I have found no way to get rid of romance bars with normal game cheats. And this is a HUGE problem when it does happen in the right situation because then sims get mad and feel cheated on and it is just a nightmare when it shouldn't have happened. This one though was accidental/glitch I believe, and i will explain why.
Following is a snapshot of Caleb on vacation with his sister and her boyfriend. They are all vampires so remember they move fast. They were all sitting around the campfire talking Lilith's boyfriend started to serenade her. At that exact moment she jumped up and ran off to check on her child. Somehow the serenade redirected to Caleb and the romance satisfaction popped up and you can see in the screenshot how the boyfriend was now a romantic interest. Lilith came back quickly and her and the boyfriend kissed and Caleb lost it, slapping him so many times like I have never seen before. That is when I took him in the house and took this screenshot. As soon as I started playing the Vatore's I took them into CAS and set their romantic interests for both romance and woohoo to the opposite sex only and turned off exploring. Even with the peculiar circumstance of how this happened this time, the game should still not have allowed a romance to initiate for a sim that has opposite likes.
Edit By Crinrict: Removed pack tag. Doesn't sound related to LS. Adjusted title a bit