Kia Ora Valerie, Good question

I guess from their perspective[some Thai Buddhists that is] to commit adultery is to intentionally make ones spouse suffer,[ adultery = Extramarital sex that "wilfully" and "maliciously" interferes with marriage relations ]...
However If one lives in a

"Free Love"

society then no adultery is being committed so there's no harm done...The other thing to take into account Buddhists don't view a marriage contract as a big deal, what I mean by this is, as long as both partner's are in agreement and do not perform actions with the intent to harm themselves or others then more or less... anything goes...
However one of Buddhism's five precepts, is to reframe from sexual misconduct

that is any sexual act that will bring harm/suffering to oneself and others...[so adultery fits this]
This sexual misconduct precept is also left open to other interpretations as well for example, some Buddhists believe "Homosexual" acts are covered by this...Their argument is if homosexuality is against local law, then it can be classed as sexual misconduct, because it violates social norms indirectly causing suffering...
But if those Buddhists who see homosexuality as sexual misconduct, remove the social aspect and just focus on the individuals then they have failed to take into account if this act is between two "consenting" adults whose intention is one of

"love"

and not wishing to harm others, then this cannot be seen as "sexual misconduct"

,
Therefore no unskilful act, no unwholesome karma created...
It all boils down to whether or not one is creating wholesome or unwholesome "karma" by ones actions...The five precepts are guidelines designed/put in place to help the Buddhist practitioners reframe from creating unwholesome karma...
You have to remember Valerie that adultery and being born transgender is only what some Thai Buddhist believe, not all Buddhist believe the same thing when it comes to karma and rebirth, in fact some Western Buddhists don't believe in either, they have dropped anything that could be seen as superstitious mumbo jumbo ...
But as for myself, from personal experience I don't "believe" in karma... I
"know" karma, and as far as rebirth goes, if I also draw on past experiences[excuse the pun], I'm prepared to give this the benefit of the doubt [or if one likes have "faith" in], and live my life accordingly ...
In a nutshell, if ones actions are designed to be beneficial to others and oneself and not to cause intentional suffering [ that is, knowing ones actions will lead to no beneficial outcomes only suffering], then they are classed as skilful actions Sorry for the long response but I have a "bad" habit of rattling on and on and on.............
Also please bear in mind I'm a lay person when it come to Buddhism and what I write is what I know to be the case...So I could also be wrong...or right...

But as the saying goes.. "Different strokes for different folks !" - Including Buddhists...
Happy Mindfulness [and many happy "
returns" of the day!

]
Metta Zenda