The only difference is, in MOST cases, the heterosexual couple is arrested for public indecency. Homosexuals are not arrested, in fact their public displays are ignored by law enforcement.
Not only that, if the gays are arrested for public indecency, they sue, claiming it was a violation of their civil rights!
No kidding. Check out how many lawsuits have been filed because law enforcement tried to shut down public sex at certain beaches, etc.