I'm not Roman Catholic, and this may not help with the rhetorical effectiveness part of your question, but I found JPII's Love and Responsibility to be an incredibly compelling philosophical account of Catholic sexual morality (which I 90% agree with despite not being Catholic myself).  Here is the edition I have and is a classic, although there is a new translation that I have heard is available.  Several of the basic arguments in this book can be made to speak to contemporary non-Catholics:

(1) I think many people today will find it compelling that one should not treat the other person in a sexual relationship as a mere tool to one's own end.  This basic Kantian point means that I cannot pursue sex merely as a way of receiving pleasure.

(2) Many people today will be sensitive to the idea that we are essentially embodied beings.  Our bodies are not something merely extrinsic to our true self (this mistake lies behind much of the transgender ideology), so therefore treating another person with love involves taking his or her embodied gender (and one's own) into account.  Thinking that you and I can love one another quite apart from our respective genders implies a kind of angelism.

(3) I think it is a fairly easy argument to make for people who are really worried about being "scientific" that reproduction must at least be a part of any adequate philosophy of sexuality.  The more we talk about reproduction, however, the less compelling many contemporary philosophies of sexuality become.

DanielCC wrote:

Ironically I find myself wondering whether the denial or points 2 and 3 can be used to push a disputant towards a non-naturalistic position on persons (or at least non-Quinean ScienceTM based naturalism). I would certainly deny both on the grounds that they verge too close to biological reductionism.

Lots of naturalists and reductionists (Parfit and other mainstream defenders of psychological accounts of personal identity) deny (2). And those folks are among the most likely to think reproduction is not relevant, except accidentally, to sexual ethics (recall Singer's dismissive comment that ethics is not about sex).

Generally, though, there is a gaping difference between taking human embodiment seriously and being a biological reductionist. That the latter is the only way to do the former... there is just nothing obvious about that. Unless one is a reductionist about animals and about life generally, the view that human persons are embodied beings does not seem to threaten biological reductionism.

DanielCC wrote:

Regarding point 1, be careful as the same point can be made about 'classic' Natural Law teachings where the end of relationships and procreation is taken to be self-perfection.

I think the extent to which this is a risk depends on how one conceives the role of perfection and of the notion of the good in intelligent action. My own view (which I take to be Aquinas's view) is that pursuing what is good for human beings is not a means to self-perfection; "self-perfection" is not a goal of intelligent agents in the same sense that "pulling a lever" or "avoiding driving off the cliff" are goals. Goodness is rather the aspect under which what's pursued is judged worthy of pursuit.

So a person having sex is not inevitably doing what he's doing as a means to his self-perfection, and his partner isn't a means to his self-perfection. That he regards it as good is rather constitutive of his pursuing it.

Romanjoe, who are you thinking of arguing with? I think that unless you know the person is very open-minded and interested in truth above all, there is little point in making a positive argument. You won't get a fair hearing. 

What you can do is point out issues and hidden assumptions in your opponent's position, such as the view sexual morality can rest on the notion of consent alone. I find that, despite priding themselves upon their knowledge and openness, modernists and liberals are incredibly unaware of their own assumptions and prejudices, which they simply unquestioningly absorb  from the zeitgeist, on these matters.