Coming inside a wet, warm and willing vag is both the most incredible feeling and indescribable.
Soft, pliably moist walls wrapping your manhood in a loving embrace. As others have said, it's a very naturally primal feeling, imagining your cum is filling her uterus and spreading your seed. Even if the ejaculation doesn't result in pregnancy or you are shooting blanks is irrelevant, as the psychological need is still there after millions of years of evolutional programming.
Imagining what she feels as your load is squirting into her is also part of the appeal as well as watching it run out of her pussy after you pull your cum-coated cock out. There is also nothing like the feeling if she comes while you are inside her, as the sense of her walls closing more around your dick in pulses will sometimes put me over the edge.
While I found I have my most powerful orgasms when I come in my wife's mouth, but injecting semen into her vagina is more superior when it comes to enhancing a feeling of closeness. PIV sex is still important to reinforce bonding and making us feel as one. I still tell her how after over 30 years together, her pussy is still amazing and never tire of coming into it.
In short:
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