I'm worried that there's no path for masochists in this kind of simulated universe (with self-modification available) to ever stop being masochists - I think it's mostly external restraints that push people away from it, and without those we would just spiral further into masochism, to the exclusion of all else.

I think we're using these words differently. You seem to be using "masochism" to mean some sort of fully general "preferring to be frustrated in one's preferences". If this is even coherent, I don't get why it's a particularly dangerous attractor.

I think normal people don't treat romance like an addiction, and those that do ("clingy") are rightly seen as creepy.

Disagree. The source of creepiness seems to be non-reciprocity. Two people being equally mutually clingy are the acme of romantic love.

We treat people who want to parent someone else's child as creepy.

I queried my brain for easy cheap retorts to this and it came back with immediate cache hits on "no we don't, we call them aunties and godparents and positive role models, paranoid modern westerners, it takes a village yada yada yada".
All that is probably unfounded bullshit, but it's immediately present in my head as part of the environment and so likely in yours, so I assume you meant something different?

(At least, it is for me. Whether or not you think it's a good thing on an intellectual level, does it not seem viscerally creepy to you?)

No, not as far as I can tell. But I suspect I'm an emotional outlier here and you are the more representative.