Well, most men are too absorbed with their own pleasure to investigate what the other side has.
Scientific research has shown that the clitoris has many, many, many more nerve endings than the glans of the penis and foreskin. This indicates that the clitoris is more sensitive and that women feel more when stimulated.
Now, scientific research has also shown that on average, a male's sexual climax lasts some 17 seconds. By contrast, that of women lasts 30 to 40 seconds.
Now, these facts alone are enough to make a man very envious but it gets even worse...
A female's sexual climax is believed by research to be much stronger than a male's. In fact some women say they have full body orgasms. Not only that, a fair number of women can climax multiple times consecutively. These women also state that each subsequent climax is more powerful than the first.
I think you can get the picture here. For men, it's as if they have some dry crackers compared to a woman's sushi dinner with desserts.
Your husband is probably very envious and when confronted with these facts, it can make him feel insecure, inferior, and it can make him feel generally bad about himself.
An interesting thing is that according to scientific research, 75% of women do not climax during sexual intercourse, and of those that do, only a fraction ( I think a third or so) are capable of multiple climaxes. So your husband may not be aware of these facts, that the majority of women do not experience anything close to what he's thinking of.
Envy is not about him not being satisfied (having enough), it's about you having more than he does (in his perception). It's about a perceived injustice (whether real or not).
Look, it may also be that your husband is a jealous person, rather than envious, (when you are jealous, you want people to not have what you have and make sure they have less than you have, when you are envious you want to have what others have). Whether he's jealous or envious, is up to you to judge, we don't know your husband.
I can't give you any solutions, just perspective. And for all the downvoters, you really seem to have problems with men being insecure. Women feel insecure (or have felt insecure) plenty and tend to receive support, yet when a man feels it he is immediately deemed worthless. Quite sexist indeed.