It does have an aspect of rewarding freedom, but he is right that it doesn't change the state of things regarding the rules of adultery in the NT.
While they aren't being very polite about it, both (Q) and phlogistician are pointing out a logical conclusion from your earlier statements: if sex equates to marriage, and a person had sex, then they are married. Given that presumption, if the Bible is Truth, then the stance the bible holds on divorce means that a person who had sex (and was therefor married) who is not currently with that initial partner, for any reason other than fornication on the part of the other person, is guilty of adultery.
Being "Born Again in Christ" means that one must give up previous ways, and that previous sins are forgiven. However, those previous sins must not be continued; previous homosexual activity is forgiven, but homosexual activity after being born again is new sin requiring further confession and forgiveness. The original "marriage" is not wiped away, as it was not a sin; any sexual union after being born again, with anyone other than the original partner/spouse, would therefor be sinful.
Within the confines of the bible, the only logical way I could see to get around this is 1) if sex is an important part of marriage, but still distinct from marriage (which you don't agree with) or 2) if the born again individual were to claim that after the breakup of the first relationship, the other partner committed adultery which is grounds for divorce post being born again. However, since the actions were mutual, this logic would seem to be a real stretch to justify - particularly if both parties converted to Christianity, and both had previous sins wiped clean.