In mainline Protestant Christian circles, Boswell's ideas are quoted constantly, even by people who may not know he popularized them. You probably see that more with his ideas about sin and mistranslations than with medieval brotherhood blessings, and of course Mainline views don't help much to counter Evangelical persecution.

Nevertheless, as entire denominations (UMC, UCC, Episcopal, etc) move toward rejecting traditionalist anti-queer teachings, Boswell is right there in the fight, and he's winning.

That may not have an immediate legal effect, but the days of Christians claiming a united anti-queer front are numbered, if not already over. I believe that will one day have profound legal consequences. Even today, when Christian thinkers try to claim that sincere Christian belief mandates discriminating against queer people and that accomodation must therefore be extended, millions of other Christians raise a symbolic hand to say, "You are misrepresenting us. Our beliefs are just as sincere as yours. We are Christians too, and we know that discrimination is not a necessary part of Christian practice."

Boswell is not solely responsible for those attitudes, of course, but his influence is substantial.