Toriel's behaviour in chapter 4 is supposed to make you as the player see her in a different light.
Kris and Susie's initial reactions speak volumes about how THEY FEEL about what they're seeing. Kris sees them, immediately lowers his head, and when Susie tries to make eye contact he looks away from her. Susie is shocked at the scene, and immediately leaves despite wanting to stay over just before.
Kris then clutching a pillow to his head, and about to escape the room until the phone rings finishes the picute.
Did Sans fuck Toriel? Probably not literally, but the scenario presented is an allegory for a single mother having a one night stand and putting her child aside to do so.
It also shows significant deviations in Toriel's behaviour from earlier chapters, especially her level of care for Kris caused by Sans' inclusion.

Does this make Toriel an evil and awful mother? No. But ignoring the fact that the kids are clearly affected by what they saw to say "well she's allowed to have friends and not have her life revolve around her kid" are broken home tier replies.