Futura,
Please proceed with extreme caution here.
First, a habit such as this (& that has been going on for this amount of time) is not something that can be just "turned off".
He may be able to suppress it for a time, but without some type of "help" (professional or personal) he is bound to go back to it at some point.
Next, I'd also advise going real slow on this whole marriage thing.
At least at this time. Why?
Because it appears the only reason he is reversing his position is because of his admission.
He is most likely afraid of losing you and thinks doing what you've wanted him to do for years, is the best way of keeping you.
Basically covering up the problem, by getting You to focus on something else.
Don't think this is the reason you'd envisioned for him wanting to marry you.
A few questions for you (should you be willing):
I'd be interested to know when the proposal came?
Did it come out at the same time of the prostitution admission?
Or did it come out later, after he saw your reaction to his admission?
Also what was his motivation for telling you his secret?
His own Guilt?
Were you about to bust him?
If not, Were you ever even suspicious?
If he told you all this of his own volition, then that IS a positive sign.
Then If he can get real help for his problem, then things may work out as you'd like.
However, if you've been together this long, what's the Rush to get married now?
Tread lightly and see how things work out.
Even if he is trying to reform, he may be trying to Rush your decision, as a Band-Aid to healing himself.
Very similar to a woman having a baby to make a bad marriage better.
Although these fixes may work in the short term, they do Nothing to solve the underlying real problem.
Unfortunately, later when the relapse occurs, its even worse then before.
And then you have the further complications of the band aid fixes to add to the misery.
Its OK to be a mate or a spouse............DON'T turn into a crutch or band aid.
He needs to discontinue with this "secret" lifestyle of his.
What plan do you have together (as a couple) to help keep him on the straight and narrow and out of his secret world?
Is he getting any IC on his own?
Does he (or do you) think he is a sex addict?
Sorry, but just looking for some additional info. into your situation, as there's not much in your first post.
Very sorry for your need to have to come here, but So Happy that you found this place.
There is a lot of great information, and tons of support here.
So stick around. It really can help (even if only to vent).
Take care