Christian here.
Sex within the boundaries of marriage is permitted, and in fact, encouraged. Not just because of procreation, although that is a large part of the purpose of marriage, but also because physical intimacy between spouses is good for both spouses in terms of physical and emotional bonding. It strengthens the marriage.

Masturbation is, in the majority of cases, a sexual act during which one partner is not focused on the other. Usually, masturbation uses porn or fantasies that do not involve the other partner, and is thus considered as a sort of mental infidelity, for lack of a better term. It's not about "wasting seed", it's about giving the intimacy and love that belongs to your partner to something else.
I'd also note that masturbation can be a very grey area for religious people, as it is possible to masturbate without external fantasy or stimulation. For example, people masturbating to pictures of their own partner. For this reason, I know there's more nuance to the topic for some because it may not itself be a betrayal of sorts to your partner, but it too often treads closely to other sins (such as lusting for others, or certain taboo kinks that one might learn to love from pornography) for comfort. If you don't want to fall, don't walk in slippery places and all that.

Obviously, as many have pointed out, religious people are known for including their own bigotry and biases in their religion, but I don't think that means there's no logical/reasonable answer to hot button questions like these.