halacha - Why does the "Dorff teshuva" of the Conservative movement say that homosexual relations are a *rabbinic* prohibition?

In a teshuva issued by the Conservative Movement entitled Rituals and Documents of Marriage and Divorce for Same-Sex Couples by Rabbis Elliot Dorff, Daniel Nevins and Avram Reisner, it is claimed in a few places that the prohibition on intimate homosexual relations are on a rabbinic level. A few quotes from the teshuva state:

We acknowledged in our responsum that same-sex intimate relationships are comprehensively banned by classical rabbinic law

^{(emphasis theirs)}

Also

We concluded that for observant gay and lesbian Jews who would otherwise be condemned to a life of celibacy or secrecy, their human dignity requires suspension of the rabbinic level prohibitions so that they may experience intimacy and create families recognized by the Jewish community.

^{(emphasis theirs)}

How do these rabbis arrive at the conclusion that homosexual relations are forbidden only on a rabbinic level? How do they explain the verse in Vayikra 18:22?

וְאֶת זָכָר לֹא תִשְׁכַּב מִשְׁכְּבֵי אִשָּׁה תּוֹעֵבָה הִוא:‏

Thou shalt not lie with mankind, as with womankind; it is abomination. (JPS)