You describe someone who's attractiveness is only skin deep. If she was not "absolutely gorgeous" would this even be a question?
"I do have feelings for her and love her son"
Doesn't sound like you love her and are staying in the relationship for her looks and her son, both reasons which will lead to disaster for all three of you.
Being physically beautiful can be a curse. Your girlfriend has probably based her identity on sex and looks because that is how people have identified her her whole life - great for attracting mates but not for keeping them. Sex naturally gets "stale" between two people after a period of time so a person who puts sex at the top of his/her list of priorities in the relationship and in life will soon cheat or look elsewhere to keep the sex priority thing going. Plus you would have to deal with the attention she gets (the curse) from everyone just like she has. Hope you are not the jealous type.
Most of all it's not fair to the boy to get attached to you and then have you leave, like all the other "dads". Mom needs to concentrate on raising her kid as priority number one in her life. And her kid will always come first to her, even if you were to marry her. You would always be second place to her child, as it should be, and if that were not the case it would be unhealthy for the child. That child will always be her son but you could always become an ex-boyfriend / ex-husband... If you want a child make your own and choose a healthy mate first - Is your current relationship keeping you from doing that? The real girl might be out there right now waiting for you...
Of course it is easy for me to tell you to dump her. And relationships are rarely "Leave it to Beaver perfect" so who knows - just my opinion.