Intermarriage
Intermarriage was the practice of marriage outside of one's culture or species.
The tribe young Chakotay observed in 2344 was, according to Kolopak, the closest thing to the Rubber Tree People still in existence. Theirs and Chakotay's tribe were descendants of the Rubber Tree People, yet never left the jungle they lived in, and rarely intermarried with other tribes. (VOY: "Tattoo")
Myleans and Talaxians intermarried. Neelix's great-grandfather was Mylean, something he revealed to The Doctor in 2374. (VOY: "Scientific Method")
In 2404, The Doctor chided Tom Paris, who expressed surprise upon learning that the EMH was married to a Human non-hologram, Lana. He thought they had moved beyond such assumptions. Tom clarified that he thought it was great and reminded "Joe" that he himself was in a mixed marriage (Human and Klingon, with B'Elanna Torres). (VOY: "Endgame")