I am not gay, so take my answer with a grain of heterosexual salt, but I have studied these issues, and it is my understanding that homosexual females in committed relationships are far less likely to engage in extra-pair copulation than are males.
Since females tend to be hypogamous, discriminant maters, it seems reasonable that two females, having found one another, would be unlikely to open their relationship to indiscriminant behavior, unless it were satisfying in some way to both partners.
On the other hand, in a male homosexual couple, both men are hypergamous, opportunistic maters, and will probably act in ways which would increase their likelihood of having intercourse with the greatest possible number of “foreign” or “strange” males, and since they understand the motives, they would seem to me to be much more likely to be forgiving of one another, and not to take extra-pair copulation too seriously.
However, in heterosexual couples, the male is pretty much hard-wired to do everything he can to diminish the possibility of parental doubt, as it is not in his interest to provision the offspring (and therefore the genes) of a rival male. Therefore, jealousy is an adaptive reaction to fears of parental doubt.
Similarly, a heterosexual female who had a child would have been at an extreme disadvantage in a feral state if she were abandoned, so she has the hardwired motivation to wish to protect the committed relationship.