Give in

The risks completely outweigh the rewards, but you're no longer thinking clearly and want to feel inside this MILF's pussy.

Moving your left hand to her opposite cheek, you subtly shift your right hand down, brushing your index finger against her labia as you pull her cheeks apart and give yourself an unobstructed view of her brown, puckered asshole. You watch her posture, looking for any cues that you've crossed the line, but besides a slight tensing of her shoulders and a lift if her head, nothing. You decide to push your luck further, massaging under the crease of her cheek while letting the tip of your index finger slip between her folds with each kneading stroke.

The stimulation to her sex has Jane damp, but her conscience won't let this continue unchallenged. "Umm, David, be careful, I think you're getting a little too close back there." she tells you.

She's apparently given you an out. It'd be easy enough to apologize, feign ignorance, and continue with the rest of the massage. Or, say "Fuck it." and keep pushing your luck. The question is, how well have you read her body language, and do you know her well enough that you can make this work?