Whether or not things fit and frequency of interactions along with libido
If both are heterosexual and healthy, things will fit.
As for libido, my experience has been that it varies wildly throughout marriage, and current libido is a very poor predictor of future libido. Time of stress, childbirth, the menstrual cycle, illness, prostate issues, impotence, menopause and andropause and all manner of factors can affect libido and trust me
will affect libido in a marriage.
It’s my take therefore, that if both are heterosexual and healthy, and are open to starting a family, and are considerate of the other’s needs or preferences, they
are sexually compatible. Anything else is gravy,
Sexual compatibility at a time when a couple is young with raging libidos should not be the foundational element of a marriage. Because, one better hope that there’s more than sex holding a marriage together; there
will be times when sex is difficult, impossible or unwelcome. For both partners. And as Murphy’s law would have it, not at the same time.