In Unfaithful (2002), why did Connie Sumner use the tissue paper in the train's restroom after having physical contact with Paul?
Ok, since it seems no one else wants to touch this one, I'll take it.
She is wiping up the semen.
After sex, semen doesn't stay inside a woman very well. Especially when she's standing, as gravity pulls it down. To prevent semen from dribbling down her leg and staining her pants (or skirt; I don't remember), she wipes it up, much like she would wipe after urinating.