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Or, are men with naturally lower testosterone levels more likely to stay in a single LTR?

ooh, also a possibility!

This is one variation of a particular line of research. Sexual orientation vs testosterone would be another good one. Getting periodic testosterone readings before long term relationships and then after would be even better to develop a trend. Locating an actual mechanism that modulates testosterone based on instinct/emotions would be even better.

My guess is that it's the former. In general, women are attracted to masculinity (assuming heterosexuality), a factor in which testosterone plays a large part.

Let them be attracted. Serves reproduction all right. Hence your evolutionary argument. But the study is about long-term relationships. I'd think social factors, cultural habits, and reflection play an important part there. It's not clear to me, that it's necessarily the former.