Very early in our relationship, my husband and I agreed that using something like "If you loved me, you would...." It's unfair to say that because someone won't do, buy, say, or agree to one particular thing then he/she doesn't love you.
This reeks of that to me. "I love my husband enough, I'm going to have a threesome for him to help fulfill his fantasy." It always reminds me of couples who use swinging as leverage - I let you do/have this, so you should let me do/have that - and I avoid that kind of situation like the plague. It breeds resentment.
I could definitely see it being an argument to try to convince someone to swing, as well. I don't know that it would be used as a judgement for ALL non-swingers, but I can just picture the couple in which the man really wants to fuck his wife's friend (and therefore let his wife fuck the other woman's husband), so he tries to convince them that if they REALLY loved each other, they could totally let this happen. Accurate? Maybe, maybe not, but it wouldn't surprise me from SOME of the people making those claims.
Others are probably just not sure how to explain WHY they are comfortable with swinging, and an inordinate amount of love sounds about right. Instead of saying trust, comfort in the stability of the relationship, etc, extra love sounds a bit nicer.