Why? that is your question.
In motorcycling there is an expression "if you have to ask - you wouldn't understand"
This can be applied to most everything - Here, on this site the majority of people, are either in the 'lifestyle' or interested in being in the 'lifestyle'. The same question can be applied, why do husbands like it when their wives have sex with other men.
Until I had my first M2M experience I probably had the same attitude as you, why? pussy is sooooo much better than dick. Like many I wanted to know, call me bi-curious, unlike many I took steps to actually find out.
Guess what, it was fun, a lot of fun, not as much as a female but a lot of fun none-the-less. Of course once I took the plunge,even once I stepped into the role of 'bi', like virginity, you're a virgin till you're not, then it's done. And unlike many I firmly believe and live the adage "IT'S JUST SEX"
So now I will suck cock or eat pussy, I don't care both are fun, one more than the other but at the same time more challenging to partake.
By the way my cocksucking has nothing to do with our hotwifing lifestyle.
One last thought for you, it's a catchphrase from the 60s & 70s - "try it you'll like it".