For those who want to have an operational definition of SMSM
From this source: http://www.glbtq.com/social-sciences/straight_men_who.html
“The SMSM phenomenon is not new, though it has only recently received much media attention. In earlier generations, these men may have been described as “trade”—heterosexual men who participated in same-sex sexual encounters of a restricted kind, often taking the active role in anal intercourse and the passive role in fellatio.
They may have been sex workers or men who engaged in sexual relations with other men only because female partners were not available (as in the case of prisoners or men who live in all-male environments). They may have engaged in sexual activity with other men as runaway teenagers living on the streets for survival or as young men out on a lark.
These men may have cruised for same-sex encounters in public parks or restrooms, but they were unlikely to become part of a gay community except on its fringes.
More recently, the growth of the Internet has facilitated the acting out of sexual fantasies of all kinds, including those of SMSM. The Internet has made it easier and safer for heterosexually-identified men to meet other men for quick, anonymous sex. Hence, there may be a growth in the number of men who experiment with same-sex desire without altering their sexual identities.”