Another kind is sex that disregards a third party's rights and feelings. Accordingly, you should respect the sexual and emotional boundaries that people agree to in order to protect the relationships that they value. Nor is it good to help other people cheat on their contracts. Even if your lover's spouse is willing to accept the fact of your relationship, her daughter or son may not. Let's start with one simple and powerful fact.
Love makes sex better.
This shouldn't be surprising. The human instincts that love is founded on evolved in order to encourage both reproduction and long-term relationships, so the kids would have time to grow up in with both parents around. In effect, human sex is designed to reward marriage. And indeed, studies on this subject seem to show that married couples generally have better and more frequent sex than single people. If nothing else, they tend to have had more time to learn each others' needs and responses.