Dallas wife said:
I appreciate the suggestion, but [my husband] is bisexual and takes real dick in his ass.
Hm... I see. Evidently your husband is, at a minimum, sexually flexible. Being bi might also mean he has a secret preference for you fucking him up his ass with a strap-on dildo (as opposed to conventional sex), but he has never said so because he also secretly feels you expect him to have "conventional" sexual desires vis-a-vis you.
If you decide to try an experiment, the only rule of thumb about size is: your strap-on should be larger than your husband's cock. Since he's accustomed to being fucked anally by other men, a possibility would be to start with a strap-on that's slightly larger, but not immensely larger, than his cock — then, you could work up to a size that's larger than most of the men who normally fuck him up his ass. The result, conceivably, may be that he'll find himself more strongly turned on by you.
Dallas wife said:
We do not have a cuck relationship, though.
Ah... but you do =p. The definition is short and simple. It is, "cuckold — a man with an unfaithful wife."
That is, since you exercise your prerogative as a married woman to take lovers (and, hey... there's nothing wrong with that), your husband... whether he knows you fuck other men or not... is your cuckold, by definition. You can think of the basic definition as the "cake."
If your husband is your self-acknowledged submissive cuckold, and you taunt him about that (because, for instance, you know he secretly wants it), and if you require him to wear lacy women's panties (because he's incapable of satisfying you; you require other lovers for your sexual satisfaction; and furthermore, because he wants other men to fuck him up his ass and lets them do so; or just because you like the idea of making him wear lacy panties) — then, all that, and more, you can think of as "frosting and trimmings" on the cake.
Dallas wife said:
[My husband] does not know I give up my ass for a select group of men. Men who COULD steal me away from him, if they wanted.
Well... what you do and don't do with your lovers, of course, is your own business.
But, you might consider that a whole lot of men, perhaps including your husband, are turned on by a woman who teases and sexually taunts them. And, they're especially turned on if that woman is their wife — even more so if they know, or strongly suspect, their wife takes lovers.
A way to carry out an experiment, if you feel so-inclined, might be to acquire a suitable strap-on, preferably a vibrating model, then... at a good time, perhaps an evening when you and your husband are home alone together, after some wine (or your preferred drinks)... go into "dominant mode," and tell him (don't ask, but tell him sweetly and seductively, whispering in his ear) to "take off your clothes, darling..." Then, when he's fully nude (but you're still fully dressed), tell him you'll be back in a moment, retire to your bedroom or bathroom (don't let him see you do this), and put on your strap-on (remaining partially dressed to emphasize, along with your permanently-erect strap-on, your relatively dominant role). Then tell him, again sweetly and seductively, to get in position... and remind him, in a taunting way, you know he's well-practiced at getting into position. Then... using adequately large quantities of lube... ease your strap-on, gently but firmly, into his ass, turn on the vibrator, and fuck him — slowly and gently at first, then more vigorously, depending on how you sense he's responding.
You may find you can orgasm yourself in this way, from the vibrations on your clit.
If your husband orgasms while you're fucking him anally, so much the better. If he doesn't cum but semen drips out of his cock, indicating you're milking him by fucking him anally, that too would be a very good sign. If he doesn't do either, but accepts being fucked up his ass by you (i.e., without adamantly refusing), then that too would be a positive outcome.
Given your husband's sexual flexibility, as strongly indicated by him being bi, you may find the outcome would be to improve your marriage. If he accepts you fucking him anally — even if he seems unsure how to take it, initially — I suggest taunting him afterwards by telling him, in your best low, soft, seductive voice, that from now on you will be fulfilling his long-standing fantasy about anal sex with you... but, you will be doing the fucking, and he will be the one who is ass-fucked.