Ahhhh sex :)
Ahhhhh, sex always complicates relationships eh ?
Just a point or two I'd toss in here.
1> Although sex is an important part of a relationship is not the "all". And often it gets placed on a higher pedestal than it deserves I think. (and I'm a VERY sexual person by nature) I just think it's important to keep focus on the fact that sex is (often) part of a "relationship" and not the other way around. When issues about sex arise they should be addressed on their own and not given more weight than any other "issue" (difference?) that may exist in any relationship.
This I think is one of the ways polyamory can help relationships grow, by, if necessary, giving the opportunity to explore meeting those needs in a wider variety of ways. Let's just take a (over) simplified example and you can expand on it accordingly. Suppose you are a person who just simply cannot engage in oral sex. It literally makes you puke ! If your partner is one who really has a passion for it then you have a conflict. Potentially. It's not right for them to force you into something that could affect your whole outlook on sex, but neither is it fair that your partner live out their life missing something they find pleasurable and special.
Now keep in mind that the raw sex may seem to have little to do with the "amour" of polyamory, but your reaction does ! If you truly love your partner - what are the options here ? What will it mean to the "relationship" by which way you react ?
So I think you can expand on this accordingly.
2> The insecurity part.
That's an ego thing, a natural reaction of all of us and something we can benefit tremendously from getting the practice putting it out with the trash whenever it surfaces.
The reality is there will always be other people in the world that seem to have characteristics that are superior to our own. Bigger, smaller, prettier, smarter, sexier etc etc. The list is endless. But if we have a relationship with someone then we must have redeeming qualities that connect us to them. Focus on those ! I'm not saying don't try to always improve ourselves in ways that we can, but neither are we expected to turn ourselves into something we simply aren't ! If we have to do that for the sake of a relationship - we're not in the right relationship. Call a spade a spade.
People have an amazing range of sexual tastes and desires - no different than we have a wide range of tastes in food etc. Don't make more of those differences than they should be. Keep things in perspective.
In the examples you gave - i.e. your husbands interest in Tantric sex etc, maybe he feels that that is just "not" your thing ? There may be things about you, your relationship, any number of things that fuels that belief. First, I'd simply ask him (in a non-confrontational way) why he believes that ? His beliefs may have some substance, or they may not. But they are HIS beliefs and we all need to respect that. On the other hand, there's absolutely nothing to be personally deflated about - as natural as that is. What if the tables were turned ? Let's say for example that your husband was just "average" in the size dept. And let's say that you occasionally need something on a (much) larger scale to really take you to special places. If you explained that to him he would be in a position to feel personally inadequate. But THAT is HIS choice ! He can bow to his ego and risk harming the relationship you have together....OR...he could say..."Well, ya know, I happen to have a buddy that is hung like a horse. How about if I talk to him ?"
And then he can slide comfortably into what's commonly called "compersion" and watch your relationship blossom like a spring flower.
It's all about choices and thinking things through rather than reacting on first "emotion".
Hope this helps....