And you're point is? You still check in on the person. Sex does involve pleasuring the other party too. It's kind of hard to do without checking in. If the other person starts to behave oddly, like suddenly doing his or her best impersonation of bambi in front of a semi, that's as good a time as any to check in.
Body language can also tell you no. Although freezing can in theory be a positive response, it can also be an extremely negative response. Freezing is not a "go ahead" signal. You should check in on a person who is freezing, because for all you know, they're mentally in 'nam.