I think orangecat's point is that rationality and the moral correctness are distinct (assuming you haven't solved the is-ought problem). "Rationality" is just taking actions which are most likely to lead to your goal, whatever that goal may be.
Now, that still leaves us to explain the correlation between (a) rationality in the pursuit of relationships and (b) having "questionable" goals like pure sex rather than love. And I think it can be explained this way:
First, sex is a much easier to define and measure goal; it's hard to gather data about how well you have achieved ephemeral things like "love". This makes it less amenable to rational attack. Second, people who are initially rational are more likely to see the rather unpleasant realities of love (tracing it back to evolution-driven competition) and become cynical of even the existence of love (and so settle for sex). Third--and I think this is close to what you were getting at--men who have been shunned by women may become so bitter that they don't really want to the love of a woman while still acknowledging that they want sex.