I need a really solid article proving our sexual dimorphism.

"You won't find one, it would be like looking for a maths journal article proving 2+2=4.."

Pedants' Corner here!
Whitehead & Russell, the 3-volume Principa Mathematica, in Volume II:

Slime moulds may have hundreds of supposed sexes, but sexual reproduction still involves two mating types of gametes for cellular slime moulds. Not sure about acellular slime moulds, is it ONE type of gamete?

But I think I prefer -- how are babies made? And where are the other gametes then?..............And doin't you just mean personality or social role then?

human

The continued existence of the ham race is your proof.

He has to prove it is not binary as he is making the challenge hypothesis.

I don't have any to hand but there are some very good longer threads on twitter that go through the issues and some that respond directly to the much quoted Nature article.

Having a DPhil from oxford - I'm assuming in a non STEM subject - and questioning whether mammals are sexually dimorphic is worse than being 'a dumb-dumb'. It implies wilful ignorance.

There's a real problem with 'the two cultures' - too many bright articulate people who have no clue, or even worse a philosophical rejection of, objective reality.

Ask him to disprove your pet Invisible Pink Unicorn. (That must be a real thing, my autocorrect automatically capitalised it).

How did he manage to get into Oxford?

Not in STEM, I assume. Probably a pomo-type who thinks it is all relative and a matter of perspective. But even pomo-academics should be able to define the words they use, no?

I think that one recent Nature paper sowed a lot of confusion because it described some mosaicisms. Still does not make Sex a spectrum, with the binary gamete (egg and sperm) being the corner stone of reproduction and any other conditions (severe intersex conditions) evolutionary dead ends. And even those intersex conditions can generally be classified as male (Y chromosome present) or female (Y chromosome absent) (sex determination). That sex differentiation can go wrong does not change that.

Any Biology for beginers book

How did he manage to get into Oxford?

Oops - x post bowl and lang

Just a personal opinion, but the phrase 'sexual dimorphism' can be a bit of a distraction as scientific studies on it seem to look at characteristics like height, musculature etc which are hugely variable. People nowadays seem to think that small blokes and big strong women prove that humans aren't strictly sexually diamorphic.

I think the key point is that humans (like all mammals and most other higher organisms) reproduce sexually -ie union of a male gamete and a female gamete.

Perhaps you can ask him what he thinks the other gametes are?!

Babies.

That's how they get made.

I agree with Bowl - just call him a flat earther and ask him to produce evidence of the discovery of a third gamete.

And if he goes anywhere near intersex, threaten him with a good talking-to by @mrkhtake2 on Twitter!

You won't find one, it would be like looking for a maths journal article proving 2+2=4, it's such a basic premise to all the research that comes after, that nobody would fund it. If someone set out to prove something so obvious, they would get endless "in further news... water is wet," comments.

If someone believes we're (like all other mammals) not a species with two distinct sexes (and the odd medical condition pertaining to sex) then they need to prove it or at least succinctly explain what they mean.

I only have access to the Abstract currently, but:
www.els.net/WileyCDA/ElsArticle/refId-a0026634.html

Sexual dimporphism is uncontested and evolution would long have abandoned it in mammals, if it was not central to the reproduction of mammals (like us). This is uncontested in Science. (I have a PhD in Biochemistry, I say that confidently).

The only point he might have is that 'woman' is not sex but gender. In which case you can kindly ask him to provide a definition (and definitions must be objectifyable and noncircular) of 'woman' other than the dictionary one, which is 'adult human female' and thus thoroughly linked to the body's sex (type of gamete). By definition.

medium.com/@tom_farr/the-wolf-of-masculinity-is-dressed-up-in-the-sheeps-clothing-of-gender-ideology-progressiveness-10ef8399ca6e
This isn't what you were after, but there is a quote from an intersex women speaking about how transwomen are using her condition as validation for themselves, and it would be hard to argue with.

Give him a good shake from me!

Also ask him why there are just the two types of gamete.

No, it’s not up to you to prove this ridiculous statement.

You ask HIM to prove how humans change sex.