Several trans females have been able to breastfeed so we are talking about real breasts.

Nah, mate. A couple of extremely narcissistic trans-identified males have claimed that by pumping themselves full of cross-sex hormones and other dangerous drugs they were able to produce some sort of milky-looking discharge from their nipples. Though these men claimed this stuff actually was milk, in fact the secretions were never analyzed.

What's more, the ex-wife of one of the men who has made these claims says that in his case what came out of his nipples was pus from a chronic infection caused by surgery to implant sacs of fluid or gel into his chest to simulate the appearance of large female breasts.

And contrary to these men's claims that they were able to "breastfeed" children, there is no testimony or evidence that the nipple secretions of these men were ever able to sustain a single child's life, promote his/her development or transfer immunity to a child the way breastmilk made by actual female people does.

Some men have sexual fetishes about lactation and breastfeeding. Some of the men with these fetishes are trans-identified. Funny that.

By trotting out the claims of these men, and trumpeting their specious claims as truth, you're really not doing your side a favor. A fella who has fetishes around female body functions and thinks that having such fetishes somehow makes him some kind of woman is only proving just how male, solipsistic and out of touch with the reality of women's lives he actually is.

Breasts are not relevant for social interactions, do you go around looking at people's breasts and inspecting them? I hope not. You subjectively value social interactions, while I, an absolute lazy introvert, do not value social interactions (though many introverts may be different from me), so I don't care about what is or is not relevant for social interactions. The issue with focusing on secondary sexual characteristics instead of genitals is that you're excluding "cis" females with less breast tissue. You literally said "cis" females with smaller breasts are less female. You're excluding literal "cis" females with less breast tissue just so you can include literal males who think they are female when they are not inside the category of female. Also duh, "trans females" are males and males can breastfeed, they can lactate, though it does not happen under usual circumstances, it does happen when males are anxious and they play around with their hormones. Hello middle school biology? https://en.wikipedia.org/wiki/Male_lactation#:~:text=Human%20male%20breastfeeding%20is%20possible,be%20used%20to%20increase%20lactation.

Human male breastfeeding is possible, but production of the hormone prolactin is necessary to induce lactation, so male lactation does not occur under normal conditions. Domperidone is a drug that can be used to increase lactation.

https://www.scientificamerican.com/article/strange-but-true-males-can-lactate/

He also notes that starvation—which inhibits the functioning of hormone-producing glands as well as the hormone-absorbing liver—can cause spontaneous lactation, as observed in survivors of Nazi concentration camps and Japanese POW camps in World War II. "The glands recover much faster than the liver when normal nutrition is resumed," he writes, "so hormone levels soar unchecked."

Males of many different mammalian species have the potential to lactate, although only one, the Dayak fruit bat of Southeast Asia, does so spontaneously

Males can lactate under such circumstances as long as their breast tissues are actual breast tissues, they can barely do such a thing when they get surgeries and put silicone or other material in there that makes things vastly different from actual breasts.