Manipulation is a healthy part of a relationship as long as it's on a foundation of love and respect.

Really, in the case of sexual manipulation (in this context), it's flirting with your partner, in the case of manipulating emotions, it's calming them down, or otherwise helping them get out of an extreme emotional state.

It becomes toxic when there is no foundation of love and respect.

If your wife says no to sex, but you manipulate, or flirt, your way into it, you're disrespecting her decision. If you instead use it to get her into the mood before the point of consent (which is usually the invasive parts; removing clothes, making out, getting on her, ect.) you're not disrespecting her, you're taking small steps to get her in the mood (with the understanding that a good partner will stop if she clearly isn't reciprocating and doesn't want to be in the mood.).

Other areas include social expectations, such as a guys disapproving of friendships with other guys, or girls with girls. It's important to have room for compromise, for example instead of flat out no friendships, friendships with clear boundaries, but it shouldn't be a red flag if someone isn't comfortable with that because the reasoning is valid. Attitudes toward and prevalence of extramarital sex and descriptions of extramarital partners in the 21st century - PubMed there are studies done which show that 50% of partners cheated with a close friend, and 30% did so with "neighbor, coworker, or long-term acquaintance" so about 80%, according to these statistics, happen with someone the other person has a relationship with, and this makes sense.

If you're having a bad time in your relationship, you probably vent to a friend, they'll tell you what you want to hear more or less and seem like a better partner than yours because they seem to get you better. If you're straight venting to someone of the same sex, you probably won't see them as a potential alternative, if you're straight venting to someone of the opposite sex, you probably will if venting is consistent. Good predictors of cheating in a relationship are essential when the relationship isn't satisfactory (The 4 Biggest Predictors of Cheating in a Relationship | Psychology Today). All this is to say it's reasonable and fair for someone to want limitations on friendships with the other sex, it's also important that they acknowledge they can't control their partner, and if they can't reach a compromise, it's best to leave the relationship rather than try to control the other person.

I think we're focusing on the symptoms of a root cause, and that was needed, and will be needed to an extent, but the focus has to shift deeper into the root cause; a lack of love, respect, and empathy that taints normal aspects of a relationship.

I'm also aware there's some external factors in the data I quoted, especially in the one about cheating, but we can only operate with what we know and take it with a grain of salt. Most cheaters probably won't admit to it in a study, but we can get an understanding for the ones that do, even if it isn't 80% on the dot I think it's safe to say, given the studies (not just that one) done, the majority of affairs of cheating happens with someone you know, not a stranger.