The big elephant in the room with the pro prostitution argument (which basically comes down to happy hooker/choice), and the punters who delude themselves with that argument, is the framing of 'consent' WRT sexual intercourse between women and men in male supremacist culture.
There are punters who don't care if the prostituted women is there by choice or not, there are punters who get off on knowing that a woman is suffering and being coerced. There are, as GiantHaystacks said, men who get off on the dynamic of paying a woman to submit to penetration that she doesn't want (i.e. rape).
There are also punters who need to delude themselves that the woman wants sex with them in order for them to get off. I have read a lot of testimonies from women who say that often these were the worst sorts of encounters as there is an emotional violation that is very damaging for them - not only do they have to submit to penetration that they don't want but they have to be intimate with a sick fuck who has deluded himself that she likes it/him and the woman has to go along with that lie.
Sex a woman doesn't want, is rape. This is the feminist analysis of rape. This is the analysis from the female perspective.
Patriarchal analysis is that rape is sex a woman does not consent to. This is a male/PIV centric perspective.
Consent can be bought, manipulated, influenced and coerced.
"Consent" is a smoke and mirrors patriarchal concept used to allow men to get away with raping women. "Consent" is a rape myth - possibly the biggest one of all.
Consent in prostitution is pay to rape.
And punters know this - they just either don't care, lie to themselves about it or they actively get off on it.
"Consent" is always the final word in the pro-porn argument as though it is an unexaminable, non-flawed neutral reality, when the truth is that consent is a patriarchal social construct rooted in the oppression of women via our vulnerability to PIV. Consent is a tool of oppression and nowhere is that more obvious than in prostitution and porn.