It is possible in some jurisdictions in Norht America, where same-sex partner marriages are allowed.

It is also possible that the other person she marries dies, and following that, she, as a widow, becomes once again a loyal lover of the boyfriend.

It is also possible to stay celibate in marriage, and this constitutes some form of loyalty, because there is no necessity to be actively involved in a sexual relationship with the boyfriend in order to be called "loyal".

There is also the situation in which the girlfriend shuns her husband sexually, and has an exclusive sexual relation with the boyfriend. This comes up sometimes with the condoning by the husband, and the condoming of the boyfriend, while the husband may or may not be informed of the details, but who gives his wife the liberty to fool around. I have known such couples. In two, the husband had been incapacitated for some time, and in the third, the husband's pennnnies was too large, way too large, to allow him entry into his wife, and they asked me to help them out in this difficult situation. I responded in kind, and set up a special collection plate on Friday Vespers.