The legal standard is not "How likely is it that pregnancy will occur?" but "Can they perform the procreative (marital) act?"
The likelihood of pregnancy could only be determined by ongoing physiological testing and surveillance, which is clearly way, WAY outside of the competence of the government. Plus, people who think they can't conceive, often do; and people who think they will conceive, often don't.
I read that in the USA there's about 180-200 births per year reported among mothers over age 50, with a third of them being over 55.
And of course, some young and ostensibly healthy couples have natural, normal (i.e. uncontracepted) intercourse for years without successfully achieving pregnancy.
So the only non-privacy-invading criterion the state can properly act upon, is "Can they do the procreative act?" Which is "Are they a male and a female capable of intercourse?"
The procreative act is "the marital act," the only act which can "consummate" marriage; to be precise, an act which deposits the semen of the male in the genital tract of the female. If you can't or won't do that, then even in civil law as I understand it, the marriage is considered as not having been consummated, and so can be annuled. It never "happened."
Two men or two women can't perform the marital act. Trannies can't either (there's either a trans-"male" with no sperm, or a trans-"woman" with no genital tract.) They can't consummate a marriage.
What about the... “We can always adopt kids”... angle?