I don't find it coincidental that this incident occurred in the same year (2154) that T'Pol became addicted to trellium-D after her exposure to it aboard the Seleya in the Expanse. According to the Memory Alpha wiki on T'Pol, her prolonged exposure to the substance permanently damaged her ability to fully control her emotions.
However, in a very Vulcan move, T'Pol commented to Trip that their sexual experience together was merely her "exploration of human sexuality" and had nothing to do with emotions. I find this, personally, unlikely since this incident occurred right in the middle of T'Pol's trellium-D addiction.
There are many references throughout Enterprise of T'Pol experiencing emotional situations--in fact, it seems to me that throughout the series, T'Pol struggles with her emotions more than any Vulcan we've seen prior to this (not helped, of course, by the trellium-D).