We already know that there are still gay people entering opposite sex marriages, at least sometimes even after identifying as gay (for example, here).
A 2008 study found that the inverse is also true: not all gay marriages are gay.
In fact about one in 10 women who entered a same-sex domestic partnership in California self-identified as bisexual or heterosexual (as did 8% of women in Vermont civil unions and 5% of women in Massachussetts same-sex marriages).
See Table 2 on page 60 [PDF] in Esther D. Rothblum, Kimberly F. Balsam and Sondra E. Solomon
“Comparison of Same-Sex Couples Who Were Married in Massachusetts, Had Domestic Partnerships in California, or Had Civil Union in Vermont,” Journal of Family Issues 2008.