Love triangle?

I would disagree. For a start you have people like Thaedydal noted that are naturally like that. I've met a couple in my time and they can genuinely have strong love feelings for two people at the same time. For others more in the middle I would say its rare enough but possible. IMHO the most common way it happens is when the different stages of romantic love get attached to different people.

Romantic love has stages, a trajectory of how things unfold. Generally speaking of course. We are all wierdos so we all vary . But generally its infatuation(I fancy him/her), attachment(we're getting to know each other), passionate love(we cant keep our hands off each other), plateau(we're still mad for each other but the crazy stuff has settled down, long term attachment(we're together and will spend our future together, but its as much practical as sexual, but very satisfying).

Now that usually happens with just one person. Or it fails at one point usually between the last two stages and the couple split and start it all over again with someone new. Now imagine, the step between the last two stages is shaky, but the person still deeply loves the cmpany and personality and even sexually the partner. Enter new person who really revs their engines passionately/sexually. They may not be true long term prospects but they could be, so the person falls into the first stage with them. They still feel deeply the last stage with their original partner. Outcome? confusion and usually they'll drop the first as the passionate stage is so strong, but sometimes they don't and that's IMHO how you can get someone in love with two people at once.