can't sexually satisfy my husband

i am new here. actually never joined a forum for any reason, but i don't know what else to do
anyway, my husband and i have been together 20 years with a seemingly fine amount of sex over the years.
i am posting in the relationship section because, although this is a "sexual" issue, it affects our overall relationship, let me know if i should go elsewhere
we didn't start a family until just 6 years ago and had our last one almost three years ago. since the last child i have had little or no interest in sex, with anybody. for example, if i didn't have my husband around right now i still wouldn't want sex.
of course, children take a lot of time, and i am happy to spend that time. things like sex are just not important to me right now.
my husband has always been highly sexed. it never seemed like a problem before. now, it seems like that's all he wants.
we are fairly open in our sexual ideas and, in the past, we have thought about being with other people, for fun. as i said, since i don't want sex, this is not my idea of fun anymore, but my husband has continued that interest and found others, online, that would be interested in being with him for casual encounters.
he discussed it with me asking if i am ok with it. i do not say no because he has to make his own decisions and i am not his mother, to tell him what to do, and i didn't worry about something that hadn't actually happened yet. i just said he must tell me when he does do something, because i do not want to be exposed to HIV.
knowing this might happen and wanting our old relationship back i have tried going to doctors and joining studies and taking vitamins, etc.
just not fast enough for him, i guess, because it turns out he went ahead and had an encounter(s)!!! so, though he is very careful, i have potentially been exposed to HIV.
putting the HIV issue aside, hard to do though that may be. how do we work on our problem now? i can't get back into sex if i can't have sex with him.
this evening he asked me to give him some relief! what is the point of having other people if one is still wanting it from the wife? i almost vomited.
i guess the reason i am here is :
if a wife cannot satisfy her husband, does she have to expect and ok him going to someone else?
should i have to even try at this point, since he has others?
i think he thinks i drove him to this, but i don't think his choice was the only answer for all men.
what do others think?
thanks for any input

I'd call it *way* more than a sex issue. Sex "for relief" can be had through "Old Lady Palm and her five daughters".

His choice (without your prior consent) to "relieve" himself outside is crude and insulting.

Perhaps he's not as clueless in other aspects of the relationship -- else you wouldn't have stayed with the relationship.

Sorry, for me, this 40-ish(?) guy seems rather immature. And now he's a father. You may need a seasoned marriage counselor to work out what's going on: with YOU as well as with him and between you.

I also think some more energy spent on discovering why "since the last child i have had little or no interest in sex" would be WELL worth it.

Best of luck. Let us know how you're doing.