There was recently an article about comfort sex.Most women don't orgasm the first few times and the number one sexual dysfunction for men is premature ejaculation so flings may not be as potentially enjoyable as a monogomous relationship. The author talks about how when you're in a long-term relationship, it's easier to relax. A long-term relationship provides the comfort needed to fully release control, get to know your partner and develop a sexual script. There's joy in knowing how things are going to happen and knowing you don't always have to put on a show. There's comfort in knowing you'll love each other no matter what.
Some people while having sex or trying something new start to engage in spectatoring- worrying about what they're doing while they're doing it. This makes sex much less enjoyable and can even contribute to some sexual dysfunction.
The best comfort sex allows you to just relax and enjoy the full benefits.