Nipples stick out (well, comparatively) as do ears, eyeglasses, ponytails, noses, moles, bandaids, etc. That makes them (by that very simple virtue) more interesting than the area surrounding them.
If your son tried to smoosh your nose all the time, I don't think you'd think "sexual???" at all. It's unusual for a normal (meaning non-sexually-traumatized) preschooler to act out sexually. Please know this. (Also, I'm not suggesting that he's been abused. I'm suggesting that pushing on nipples when you have the chance is fun for a two-year-old.)
How should we deal with this behavior?
First, I want to re-emphasize it's not sexual, nor is it a particularly irritating behavior (in my opinion, anyway.) Deal with this behavior the same way you would deal with any innocent (but unwanted?) behavior. You could:
- ignore it completely (your lack of a reaction might make it less fun for him.)
- gently but consistently say "no", remove his hand, and cover your chest/ wipe your face
- distract him with something more interesting (start telling him a story, or reach for a toy for him to poke; don't make it too obvious or he might associate pushing your nipples with a fun reward!)
- give him other toys that squeak, or have eyes that bug out, or do other fun things when he pushes and prods them, and perhaps toys he can safely put in his mouth?
- teach him to kiss instead of suck on your face