Sex as a transaction is problematic. Sex as a gift, freely and mutually given isn't. Those involved give and get qualitatively similar outcomes.
So swapping sex for money, security, intimacy or affection are all problematic to an extent, just to different degrees. Some of those things are ok if they are a minor part of the overall relationship, but only if it's a genuinely two way street- I.e. Both partners enjoy the intimacy and affection involved in sex, rather than one person craving affection and the other craving sex. Two people giving sex out of love, affection, friendship, respect etc- less potential for problems, less potential for hurt.
People see a variety of things as exploitative when both/all partners aren't responding in kind. The important bit here is responding in kind, rather than swapping/trading/transacting. It's fundamentally an act that is not about not about entitlement or contracts, but about free choice at the moment of choice.
For example, A regularly has sex with B. B is in love with A. A is not in love with B, and knows they won't fall in love with B, but it's a convenient way to get regular sex. Many people would see the asymmetry in that situation as problematic.
A and B regularly have sex. They aren't in love but enjoy sex with one another. Many people don't have a problem with that because it is symmetrical. As soon as it might become asymmetrical it becomes more problematic. And it often does e.g. one of the friends in a FWB situation falls for the other and suddenly something fun becomes hurtful and problematic. Many people don't believe in FWB just because there is a chance that it can become asymmetrical.
People speak about the emotional strain having sex purely to conceive can bring if it feels forced or artificial. This demonstrates how it can be difficult to have sex for any reason other than enthusiasm for sex with that person at that moment, however shared or pleasant the end goal might be.
Men who have sex with protitutes are men who don't mind exploiting other people. Being exploitative is not a good character trait in most people's eyes.
And people who freely and knowingly choose to enter into sexual relations with people who are in marriages/relationships- also not a good trait in many people's eyes.