If you're not getting enough fat hatred and toxic messages about marriage being the ultimate goal for all women everywhere in your romantic comedies, then here is Good Luck Chuck to fix that for you!
Random Female Character: You're a lucky charm! You have sex with someone, then they find their true love. Isn't that how it works?
Male Protagonist: You know, that may be so. But while I have no issue with recreational sex, whether in general because I don't judge consenting adults or specifically in my own personal life, the fact remains that I do take issue with and am personally uninterested in sex where I would be actively being used for this supposed benefit, nor am I interested in having sex with women who do not genuinely want to have sex with me as a person as opposed to wanting to have sex with me as a concept, i.e., a "good luck charm".
Male Buddy Character: Still, don't you think you should test this on a totally undesirable fat woman?
Male Protagonist: No. The experiment would be meaningless, since I reject the idea that any human being is unlovable simply because they do not conform to mainstream standards of beauty, so if she married after having sex with me, that would not be proof of anything. Nor would her not marrying be an indication of anything; it is toxic to assume that all human beings desire a sexual soul-mate, or that such a discovery should universally result in marriage.
Furthermore, I am completely contemptuous of the idea that I should use her sexually in order to test something about myself. Even if she consents to the sex itself, my decision to use her as a litmus test would be deeply dehumanizing to another person, and would make me profoundly grody. I would rather live my life choosing to respect others and expect respect from them in return, than to use others and to encourage them to use me in turn -- a choice that I think more profoundly affects my worth as a human being and my chances of finding a soul-mate (if that is, indeed, what I desire) than anything else.