The context is important, in this last scene with Oberon she said "her heart never be his" and only complain the way he call her pussy a "wet little hole" so she is at least resignated in having sex with himAnd we have little information about what happened with Oberon. We saw that he groped Asuna's breasts and we have Kirito's dream where they fuck, but how true can it be? Well, at the beginning of the first part, Asuna, although she does not finish with Kirito, but from the scene of their sex it is clear that she likes everything. So perhaps Oberon is not far from Kirito in size