There are a couple of daddy kings out there in the world; those who embody the uber-masculinity and are known by straight women and non-straight men as daddies. The ultimate daddy is probably Henry Cavill, aka Superman. He's brawny and muscular, he's a protector, he looks like he could break you in half. The late Luke Perry was considered a daddy too, as is Jason Momoa, Idris Elba, John Hamm, Chris Hemsworth... you get the picture. They're stereotypically "hot" square-jawed men around 40 years old that are equal parts father fantasy and fit guy reality.
The female equivalent of the daddy is the MILF, which I can't spell out for you so you'll have to look it up. This term was popularised after the release of American Pie in 1999 with the character "Stifler's Mom"; an overtly horny and seductive housewife in her mid-40s who was eager to have sex with younger men.
Yet calling somebody a MILF hasn't really been seen as a compliment over the last 20 years. In fact, it's been coded (rightly so) as misogynist. It's a trope used by boys and men for their porn searches and little else. Daddy, on the other hand, is a compliment that men are taking on and owning.
That's because the daddy has been hypersexualised instead of being demeaned like the female noun counterpart. In the porn world, "who's your daddy?" and "yes, Daddy!" are common phrases to hear. They denote a male being worshipped as a father figure. Perhaps (and I'm just guessing here) it's a manifestation of a fantasy one may have once had about having sex with their friends' dads when they were first coming into their sexuality as teens?
Personally, I like being called "Daddy". Some of my friends use it jokingly on social media when I post a "hot" picture, and it has been used leading up to (and during) sex too. It feels like an appreciation of the fact that I'm ageing well, and dissuades all malaise about being a younger, biologically more virile man.
As long as you can suspend belief and not connect the word "daddy" with your actual father, it's a healthy expression of desire. It is fun to call someone, or be called, Daddy, because it's a term of endearment that heightens sexual atmosphere and increases intimacy.
However, it is problematic when used to infantilise one partner into submission. When your fantasy is illegal or perpetuates rape culture, it's no longer a salubrious sexual manifestation but rather something more sordid that's quite unacceptable.