Is it normal to prefer solo sex?

Glad to hear you can't be offended. I respectfully disagree with your assessment of SFPs, and there are some brilliant philosophical fallacy arguments to that effect. First, humans are evolutionarily positive animals, even when objectively facts do not support a positive perspective. That we tend to believe certain things says nothing about the empirical mechanics impacting the manifestation of those things.

I also believe the reason pop psychology references SFPs where "doom and gloom" is concerned is because it's easy to assume that a negative social interaction results from an individual's supposed negative mindset, even when that supposition cannot be rigorously verified. But if SFPs were valid, positive outcomes from positive thinking should also be ascribable to SFPs. I don't see any empirical reason thinking you can't should be any more effective than thinking you can, other than humans' penchant for speculation, especially as it regards the desirable element of control.

I see far more guys who believe they're in control of their game who either don't ever get girls (that any of us ever see) despite their effusive self confidence, or who eventually settle for girls they're not interested in (disastrously for them and the girl). And among the guys who don't get girls, contrary to the fluff about attitude and effort, there are other objective reasons they happen, statistically, to remain single. We're not supposed to discuss these in our culture, but that doesn't make them any less potent.

We can agree to disagree about SFPs. There is very little empirical evidence substantiating them, and what social psychology has to teach about the effects (or even roots) of positive (or negative) thinking on outcomes, on further inspection, reveals behavioral traits linked to neurological development that has its roots in prenatal and early childhood development, yet which very strongly affects affect throughout life. For many, affect tweeks just don't work.

Apparent SFPs are also far too easily confounded by other social and psychological variables so that what appears to be the reason for many negative social outcomes isn't, as often confirmed by further research.

A good quantitative assessment of the fallacy of SFPs rests in the aphorism you've offered: "Whether you think you can or you can't, you're right." Obviously, that's not true. There are countless circumstance in a competitive environment in which individuals are confident they can do something yet fail. And there are many, many situations in which someone doesn't believe she has a chance but wins due to significant objective superiority. Not only do we see sociologically that there are important hidden variables that impact success/failure odds, but as an endeavor becomes increasingly competitive, it's obvious that confidence isn't going to be among the chief determinants of success. Otherwise being very confident would significantly improve the chances of winning a coveted internship or job via interview.

SFPs appear to be one way to rationalize outcomes we don't really understand rigorously. Too often, when tested in medicine and quantitative psychosocial experiments, the SFP argument is debunked.

Lastly, I didn't say "I've been let down ..." I said that I am glad I enjoy my own company and have taken care of myself better than others can take care of me. There's no setting myself up for something negative in that. If anything, I'm setting myself up for joy because I am not dependent on a partner for sexual satisfaction.

If you believe what you read around here... no.