Some excellent points were made about that here:
To summarize some of the best:
- Players don’t want to feel like they’re interfering with someone’s OTP, so if the characters ends up with someone else if you don’t date them and it’s a great match, they might be reluctant to go after them in the first place.
- It might be better for player ego, and the MC’s central role in the plot, to suggest that they only pursued another romance because they were told by the MC it wouldn’t work with them.
- “Pairing the spares” can work, though, because it makes ROs seem more like real people and less like they were waiting to be romanced, and go back on the shelf if they’re not picked - that’s not how people act. It also might help players who feel unhappy about rejecting anyone, to know they have a chance of going on to a happy relationship anyway.
As for the orientation thing - I think you have a point, and the character should stick with whatever orientation they presented to you in their future relationships. People in the linked thread mentioned being happy to see a gay RO, e.g., go on to another same-sex partnership, because it underlined they weren’t just “gay for them”. But also you raise a valid issue about it being potentially upsetting to see a character you liked with someone you wouldn’t have imagined.
It’s hard enough in real life for someone to see their ex with a same/opposite sex partner if they didn’t expect that from their history… and yes, it can work both ways. I’ve had plenty of lesbians reject me because they found it absolutely disgusting that I was also attracted to men. This is one of the reasons it’s helpful to distinguish between “MC is attracted to male ROs flag true” and “MC is bisexual”, or likewise for the characters - being bisexual can change some things.