I’d guess that more honest reporting (less social stigma or shame) accounts for a good bit.
For quite a while there was a wide discrepancy believed to exist on how calories affected men and women, until it was realized that women much more widely under-reported how much they ate. As much as men may have been stereotyped as being more widely unfaithful in the past, they were overwhelmingly doing so with women, who apparently didn’t count in the stats somehow (not homosexually).
But other factors could include that it is just easier nowadays with tinder and a general hookup/one night stand “dating” culture. A decline in religious observance and moral education has its effect as well.
Also, with more women working and making more money, they have more means to support outside activities, have more interactions/opportunities, and face less severe financial consequences if the marriage collapses from their infidelity.