Why does my boyfriend prefer masturbating then sex with me?

This is a situation i have never been in, Ever. i am in my 30's and he is in his 40's we have been living together for almost 2 yrs. in the beginning our sex life was good, a little bland but it was good, however , for the last 6 months he has maybe had sex with me 3 times, the other times he masturbates, I work 40 hrs a week sometimes a little less, he works 50 plus. when i come home from work 3 out of 7 days a week he will lock the door when he is at home when he would never do that before. i knew right then what he was doin. he don't try to hide it the evidence is in the laundry basket, I have grown obsessed with this, i check it everytime i come home, i have asked him why does he do it but not have sex with me, he tells me cause he is tired and he works alot. ok, but how can he be tired from work but come home and masturbate but if it was my day off work, why can't he have sex with me. its pretty much the samething right?. the last time we had sex was 2 months ago today. i don't want to nag at him about it cause nagging is not me, but i can't help but feel as if i'm the one to blame, should i be worried, or is it something that men do, cause i am aware of the whole masturbating thing, it's natural, but come on seriously, its been 2 months!!!! thanks and please help!