First off, Friends came much later than Seinfeld and as TV gets more lax with what's shown on it, things are naturally more risque. Seinfeld dealt with relationship stuff but it was almost always in a humorous and comical light, whereas Friends had a lot of story arcs involving more serious relationships, and there was a lot more innuendo used I feel.
One may argue that it might not be as easy to identify whichever differences that both shows contribute to sexual content but being that I have to choose I would say Friends probably because it contains more female main characters by far when compared to Seinfeld at least and Seinfeld is more about everyday comedy and Friends is more about relationships.
I think the reason being that it was to make Joey look like a buffoon, and Italians in general. Think about it, Chandler was the noble white man on the show, and Ross was the noble Jew. Joey on the other hand was a caricature of Italians, oversexed, overhyped. At least 90% of the sexual content (which wasn't explicit at all) came from Joey. He made jokes about girls. It was to make Italians look stupid and to further fuel anti-Italian prejudice fresh in WASP society's mind.
"Seinfeld" still was way more R-rated. There was less sexual moments on "Seinfeld," however any of those moments on "Seinfeld" were way more explicit than "Friends." Orgasms (I don't remember "Friends" having it). Joey was a foil