A few years back on the show, it seemed that Howard always would find a time in the show to crow bar in a story about him having sex with Beth the previous night. While I don't listen as closely as I used to, I don't recall him mentioning having sex with her in a very long time. On the other hand, a recurring thread on the show lately is him jerking off while Beth is away and Beth seems to be away A LOT.
He seems more excited about jerking off to porn than to having sex with his wife. I can even imagine his grotesque naked body peering by the window as Beth peels off to a "party" or "to meet with her personal trainer" all the while Wiggy giggling like a school boy who's parents have just left for the weekend. Most of the porn he watches is of teenagers having sex with men his age so perhaps Beth has gotten too old for Howard to feel attraction to anymore.
At best I am guessing that they do not have sex more than once a month and I have no doubt that Howard is thinking of the girls from the kiddie videos (and may be even thinking about men) during the rare occasions he does have sex with her.