Drawing such a generalized conclusion from a study based solely on photographs would be methodologically flawed. To reach such a conclusion, a group of single women would first be asked to choose between men in pictures described as married or in relationships, based on their perceived attractiveness. Then, a second group of women would be asked to choose between men described as single and married (the same men described as married in the first group). If, in both cases, different women chose the men described as married as attractive, then perhaps it would be more plausible to conclude that being married is considered more attractive than being single.
If it turns out to be a fact that married men are considered more attractive by both groups of women, then it might be logical to evaluate this primarily from an economic perspective. In other words, it is always possible to say that increased demand increases value. However, this cannot be the only explanation.
On the other hand, as many women frequently express today, the view that 'men with desirable qualities and attractive attributes are quickly preferred (snatched up)' may be gaining traction, and the belief in the presence of these qualities could be a factor in women finding married men more attractive.
It is sometimes observed that a group of women who enjoy competition and measure their self-worth by the amount of their attractiveness turn to married men to test their competitiveness. However, even if these women achieve their goals in such a competition, it does not mean they have won the race. Because in most cases, the married man's wife is not even aware of such competition. Moreover, this is not a fair competition. Because the single woman who is with a married man has the luxury of being able to decorate the limited time they spend together as she wishes. She often doesn't have the problems of childcare, dealing with extended family issues, or buying a new washing machine. Also, it can be misleading to think that these pleasant experiences, experienced in limited time periods, will be lasting in a long-term relationship.
Finally, it can be said that another motivation for some women choosing married men is their preference for being in a relationship that reduces responsibility. In other words, being together may be important if one tries to stay as far away as possible from feelings of possessiveness or being possessed. However, the expectation that a relationship with a married man will be less problematic and less responsible may not be realistic.