Legal prostitution would have an impact on the institution of marriage and family. Marriage creates an environment based on exclusive consent for sex. Legal prostitution would change the nature of exclusive consent, since it is would be legal and socially acceptable. Then, based on the law of supply and demand, legalization would increase the supply of sexual workers, thereby causing the prices to fall. This further erodes exclusivity even among the pros.
Men still pay for sex in marriage, but in indirect ways, such as with gifts, jewelry, romantic gestures, and even bringing home a weekly pay check. Legal prostitution could cause some men, to shop around, for a better value in terms of sex.
Picture a husband, who decides to get some of his extra sexual needs from a pro. This sexual arrangement is very straight forward, compared to the romantic games that may be needed, after a few years of marriage. With the pro, there is not a lot of time wasted playing games. The price is straight forward and does not change with moods and conditioning strategy.
This professional simplicity for sex would change the dynamics between husband and wife, making the wife less influential in terms of sexual leverage. She will have to compete with a pro, who does not have to put up with her husband, day after day.
The net affect is this would benefit men more than women, and further cause a decline in marriage and the nuclear family. The reason prostitution is illegal, in most places, is due to women. They understand this would not work out in their favor, in the long term.
Guys typically want more sex than their wife, There is a steady decline in sex, after the first year of marriage, with this increasing sexual deficit, part of female leverage in marriage. If a pro fills in the deficit, the leverage is less, and the rules of the marriage game start to change.