lol fchan discussion.
I can imagine that the Pokemon world treats Pokemon as intellectual equals who aren't capable of voicing their opinion, and as such, there will be measures in place to protect against Pokemon abuse [and you know exactly what kind of abuseI'm talking about]. However, I don't think that there'd be too much of a ruckus if the romantic/sexual relationship was with a Humanoid Pokemon (not necessarily the Humanshape egg group). People may not talk about it much, but they'll accept it's there.
If the same type of relationship was with a non-humanoid Pokemon, then yes, I can imagine that the neighbors wouldn't be very happy, but I doubt there'd be legal issues unless the Pokemon was being abused.
And for all you people BAAWWWWing over "what people do to Ditto", just remember that Duplica's Ditto could turn into a human. What do you think she did when she got all ronery living by herself?