It's not about competition at the sperm level, gene quality as a whole. An individual is attempting to maximize its own genes. So a woman mating with a more masculine man will give her offspring the advantage of having "alpha" genes. But alpha men are not usually strong providers as they have their pick of women and so they spread their genes widely and divide their resources (or withhold them altogether). Thus maintaining the solid provider as the adoptive parent makes up for this. Of course, a man (evolutionarily speaking) has no interest in raising another man's kids. So the woman has to be sneaky with the affair. If the deceit remains hidden, this setup is by far the best the woman can do.

As far as having kids who are themselves not good providers, an alpha man's strategy is usually one of spreading his genes far and wide. If you're desirable enough, that may be a winning strategy for you.