Keith Burgess-Jackson offers six items of sound advice, each both prudential and moral. Here is #1:
Don't touch a woman without her specific consent. Consent, to be consent, must be informed. Don't resort to trickery, subterfuge, dissimulation, or manipulation (including getting her drunk or high).
I would add a qualification: unless she is your wife and you have a loving relationship. (And if you don't share a loving relationship, why are you still married to her?)
That is a good point about the nature of consent, by the way.
I would say that the above, even with my qualification, is a rule of thumb (rule of hand?), which is to say that it is broadly accurate.
Suppose you are seated opposite a woman not your wife whom you find attractive and with whom you are enjoying a tête-à-tête. She strokes your hand. She has given you a signal that may justify your touching her in a similar way even without explicit consent.
Prudent? Moral? The near occasion of sin? You decide!
In any case, don't be an Al Franken. Where is his hand? And what can be inferred from the gal's 'goosey' expression?