Very hard to read...
Men are HORRIBLE communicators. I tend to believe the opposite of extroverts and introverts however. I think that often, people who seem extroverted are the ones who hide true feelings behind alot of talk. True intoverts (which is probably me) will talk ONE person to death and hide true self to everyone else.
My husband is much like yours. He NEVER tells me how he feels until I practically BEG it out of him. However, when I do get him to open up (after he gives me the rigamoro of "I dont want to hurt you.") we tend to be on the same page. I think your husband probably feels the same. No matter how open and accepting of his feelings you are- he will not want to tell you how he really feels because he's afraid it'll be the wrong thing to say. If he accepts your girlfriend- he may be sacrificing his security, if he does not, he is sacrificing your feelings.
My heart BREAKS for men really. It must be so hard to bury your feelings. I told my husband that my heart hurts that I am preventing him from experiencing all of the sexuality and intrests he may be wanting to have... to this he replied "but I would never cheat on you." NOT THE POINT. The point is... I love him enough to want him to experience EVERYTHING- I don't want him to resent our love. BUT... how can I do this and feel secure if he can't even express what he's feeling??? URGHHHHH.... I think you'll have to either drag it out of him or, just do what you feel is right for you.