Good guess. They've done this study, and you're 100% right. We watched a bit of a film discussing it in my anthro class. (I didn't note whose study it was, but the film is called "Why Sex?" and you could probably find out from there.)

They used a program where you could slide smoothly between a very feminine face and a very masculine one, and asked women to find someone along that scale who looked ideal for a short-term fling, and someone else for a long-term relationship. The difference between the two follows the pattern that you'd expect--more masculine and virile-looking for the short term, softer and more kind-looking for the long term--but both answers slid further towards the masculine end of the scale when the subject was currently ovulating.