there is not enough material at this point. It serves to create the setup for future good action and tension I hope. But at this point I can tell you should wait. The couple only put words on what they (she) want to do and boundaries (so few).
The GF is very sexy in regards to the MC. So I hope swinging will push them far in NTR and cheating with MC left over. I'm not so much in NTR but here the setup seems perfct to mec as the girl is so much sexier than him.
I hope the other couple will use the GF as their pet for they own sex needs, with the MC allowed to fap aside the bed. The only thing I don't want is the "bigger dick" trope as it is far from reality. There are so much things other than that to motivate cheating (money, resources, power, bisexuality,beauty, ..)