In the beginning things were great; 6 months ago they started changes. No sex life unless I ask for it, make excuses; moody & mean sometimes; defensive; show little to no effection to me; a woman sent him naked pictures of herself; seems nervous when questioned about certain things

Sounds like he's moving away from you emotionally and sexually. Have you tried to talk to him about what's going on in a neutral, non-defensive manner?

He may not want to be in the relationship any more, and doesn't know how to say it - try giving him the opportunity to open up and talk about what he's feeling.

At least you'll know one way or another.

Have you asked him why, honestly? Express yourself to him.

What are the other "certain things"? That makes him nervous?
How did you find out about that naked picture? Did you ask him about it?

Communication is key. Honeymoons don't last long. Then its time for reality.
(the reason why you are together in the first place) And if they are good reasons.

Sounds like the honeymoon is over for him, got what he wanted.
And now just gliding.

Relationships take work.

Get it out on the table & decide after.

If you guys can't communicate, then say later, skater.

How old are you guys?

Well maybe he isn't interested any more.. talk to him

I have asked him several times about this, told how I'm feeling and he says nothing is wrong. He lives with me, so I told him that if he weren't happy here with me anymore that he isn't obligated to stay. He knows that honesty is so very important to me. I saw the pics on his phone, I have been looking for signs that there is someone else, but he says I don't have anything to worry about. His ex cheated on him 3 times and he says he would never do that to anyone because he knows how it feels. I don't know what to believe because his actions & words don't match. I don't accuse him of cheating, I just try to tell him that things have changed with our sex life and I miss him. He makes excuses that it's because his body is changing, he's not as active as he use be. He can't keep it hard during sex sometimes and he gets so frustrated, but he won't go see a doctor about it. I'm trying to be understanding but I like sex and I'm very horny all the time because of my hormones.

Apparently, you're not longer attractive to him.

Why don't you try and spice it up a bit, and do sexy little things for him? Maybe he's bored with just sex?? It happens.

You've only been dating for 6 months. I know it seems like a long time, but in reality, its not. Its OK if this didn't work out. That's why we invented dating... to find our match! Apparently, you two aren't a match. Its nothing personal towards you or him. Sometimes these things just happen.

Talk to him! If he doesn't want to talk to you.. MAKE HIM. You deseve to know where you stand in YOUR relationship with him.

If all else fails.. Walk away with your head held high... at least you tried :)

Things change in a relationship, and the lust wears off quickly sometime and when there is nothing else to keep the relationship going then the sex suffers.

Healthy really loving couples make adjustments to other areas of the relationship, that can sustain it through all times, but since you have not built that type of connection, understanding, or honest communications, but live together after 6 months, then I have to say that things have run there course and there was not enough there to make either of you happy.

Sorry, but lack of sex is generally only a symptom of other problems in the relationship, that need to be addressed.