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Could be psychological.

For example the numbers 6 and 9 could subconsciously remind you of a penis.

The number 3 could remind you of testicles. This could come from the Preconsious (outside of conscience that you can become aware of) or Subconcious (below awareness but can effect you) depending on your awareness.

(I'm not trying to be mean or anything.)

It's actually something called Stimulus Generalization.

It basically means that because these things remind you of something sexual they can elicit a response of sexual arousal.