Yeah, he got a lot of flak for it at the time.
The Guardian took its usual approach:
Booker-winning author appears to question people’s right to decide their own gender
www.theguardian.com/books/2016/apr/02/ian-mcewan-criticised-over-transgender-remarks
In a Q&A after his speech, one woman asked McEwan, 67, to clarify what she called his offensive remarks, the Times reported (paywall). “Call me old-fashioned, but I tend to think of people with penises as men,” he said. “But I know they enter a difficult world when they become transsexuals and they tell us they are women, they become women, but it’s interesting when you hear the conflict between feminists now and people in this group.
“It’s quite a bitter conflict. Spaces are put aside, women are wanting to put spaces aside like colleges or changing rooms, and find from another side a radical discussion coming their way saying men who want to feel like it can come in there too. I think it’s really difficult. And I think there is sweeping through American [university] campuses a kind of strange sense of victimhood and a sense of purposeful identities that we can’t actually all of us agree with. Of course sex and race are different, but they also have a biological basis. It makes a difference whether you have an X or Y chromosome.”
"Stonewall condemned McEwan’s “uninformed views” as “extremely sad”. In a statement it said: “The complexity of gender identity extends beyond genitalia. Trans people need and deserve acceptance and equality. This sort of commentary doesn’t just denigrate the trans experience, it denies its very existence, and that’s especially hurtful for a group of people who have spent their lives fighting to be heard and understood.”
"Human rights campaigner Peter Tatchell described McEwan’s stance on gender as a form of “ethical authoritarianism”. He said: “There seems to be a league table of oppression, where some people fight other people to claim the title of most oppressed. This disempowering holier-than-thou rivalry was never what identity politics was supposed to be about.”