The time-old defence that "people pay for sex somehow" has probably a lot more meaning than people might think. It goes beyond the literal meanings (ie. having to buy your partner gifts, food, alcohol etc) and the metaphorical (the emotional costs, support etc) and I think goes somewhat to a deeper level. There are other metaphorical costs involved in a sexual liaison, although these are somewhat less apparent.
The one that springs to mind first is that of having a one-nighter with an attractive girl or having a sex-buddy. Although this is generalising here, when it comes to a sexual encounter with an attractive girl, the price is that you must be of a sufficient attractiveness to meet her needs (although the effects of alcohol somewhat drop this price in exchange for the price of getting her drunk enough). This is not really a price that one can "save up" for, you're either attractive enough for her, or you're not. So you go for someone less attractive for an intimate encounter, the price? The perceived "lowered standards" needed (or the cost of more alcohol for yourself!)
Of course, then when moving into a relationship, that increases the cost substantially for the sex, and many people pretend to accept the cost in order to satisfy their sexual needs, only to default on their payment when the relationship aspects are called upon by the girl. The ultimate price for this kind of sex? The girl hates you for using them and has a re-enforced perception that all men are just after sex.
Then of course, there are moral costs too. These come in all forms such as taking advantage of someone (intentionally or not), starting a relationship for the wrong reasons, getting involved with someone you don't want to on the whim of an urge, the list goes on really. So in the end, the idea of paying a W/L is quite simply taking all those hidden metaphorical costs and transferring them into one main literal cost. It removes the physical attractiveness costs, the emotional costs, the relationship costs, certainly the alcohol and moral costs (although others would debate the last one) and transforms it all into one up-front payment. But for people who are metaphorically poor in their abilities in those areas, the cost of a W/L probably a lot cheaper and certainly less costly on their wellbeing in the long-term.