Ariel wrote:"If the angels are not given in marriage, it follows that they are not the type of beings to marry at all. It would be cruel of God to create a being with a propensity to/capacity for marriage and then deny it of them."
The text doesn't indicate that the angels' purpose, if indeed they were angels, was marriage, nor that marriage was a God-given propensity. It's quite possible those angels had some other reason for wanting to inseminate human women. For example, judging by the outcome, they may have wanted to start a line of supermen to compete against, to rule over, or even to eliminate natural humans.
You misunderstanding here is due to your misunderstanding here:
"All that is to say, if angels are not given in marriage, it seems to follow that they do not have the natural capacity for reproduction."
That seems a non-sequitur. If God never planned for angels to marry, they could still have the capability for procreation. In fact, we see numerous places in Scripture where angels assume human form, as I'm sure you'll agree; if they can do this unnatural or supernatural act, what stops them from procreating? Perhaps some women "show[ed] hospitality to angels without knowing it" (Heb. 13:2).
I take it you would agree that if it
does follow that "if angels are not given in marriage. . .they do not have the natural capacity for reproduction" then my argument is sufficient to prove its point (if not, then you're only painting yourself into a corner, because once I prove that it follows and you deny its sufficiency after all, you'd just be guilty of moving the goalposts, a logical fallacy). Granting that, the only issue is why it should follow, since it obvious that angels are not given in marriage.
The reason is simple enough: the purpose of marriage is procreation and the raising of children. Put differently, marriage (by nature) is a
sexual union, which is to say, it is a
procreative union. Any beings who marry then are in principle capable of just such a union. A being that cannot engage in such a union cannot be married (By the way, that is why the whole notion of "gay marriage" is self-contradictory non-sense, to put it politely. You may as well speak of square circles).
We cannot simply say, of course, that the reverse is true, namely, that any being that cannot engage in marriage cannot have a sexual union. That's obviously not true on a lot of levels. But that doesn't affect my argument. In fact, when we look at the exceptions, we see that it proves my case. Animals and plants do not "marry." Some may mate for life, but that's hardly the same thing as a marriage. In fact, that we know of, the only beings that marry are humans. The reason is that marriage is not
only a sexual union, but it is also a public union. That is, it is the basis of family, which is the basis of society. In still other words, there are good reasons that
society has traditionally demanded sex and marriage (or, at least, marriage and children) go together. Once again, animals may be social, but they do not have a society--a civilization--in the same sense that humans do.
So what we see from all this is that rational creatures perceive the end of sex is procreation and that children of such creatures demand a mother and a father in a lifelong commitment to one another, which is to say, marriage. That, by the way, is the bottom line of the rather standard natural law argument for "sex-only-in-marriage." But on this, then it is obvious that any intelligent being--or at a bare minimum, any being that is going to reproduce
with a human--must have the capacity for engaging in marriage, for where sex is among such beings, marriage is. But, modus tollens, where there is no marriage, it follows necessarily that there is no sex.
Now, in all of the above, I am very much assuming a natural law world view. If you are not familiar with it, I would highly recommend you read Edward Feser's
The Last Superstition. Other than that, I would only add that our culture's general rejection of natural law is near the root, if not the root, of our cultural degradation.
edit: bottom line - the capacity for procreation necessitates the obligation to marry where it [procreation] occurs. Where there is no marriage, there can be no capacity for procreation, for that would be suggesting that the being possess a
natural (which is to say, good) end that it would be necessarily and always evil to meet, which is contrary to the meaning of
natural.
edit2: it's really disturbing that you take Heb 13:2 to refer to women having sex with angels.