I just think this wrong. Marriage isnt just about sex. It is about fidelity, love, support, trust, emotional and physical well-being. When you marry you commit yourself to that one person, and any children that come along. (And i dont mean that if it all goes wrong you should stick it out because you are "married")
Say your DH agreed, and your lover felt more for you than you did for him, what if he then becomes your stalker? What if he tries to force you to be with him by taking your DCs? Or hurting your DH? What if he killed your DH and your DCs? Or if he killed YOU? How would they/you feel when the shit hit the fan then?
It would destroy you, your DC's respect for you, and your hubby for agreeing in the first place at the very least.
If your sex life is that boring buy a vibrator!!!!! Dont bring someone else into the mix. Try and spice things up again with your hubby, play some games, buy some things - doesnt have to be expensive just fun!!!!!
Either put everything you have into trying to make your marriage work (no one ever says it is easy), or if that fails then walk away, play the field as much as you like then...