I do not know what their motivation is. I would guess that they want to be as much like women as possible. I do not think that the motivation isn necessarily sexual and nobody has provided evidence to suggest that it is (I accept it could be in some cases).If you are claiming that the person in the article who said it does no harm is wrong, fair enough, but I would like to see some evidence.
I can't get into the mind of a trans person. I have no concept of what it feels like to thin k you are trapped in the wrong body. Frankly, I think calling it a sick perversion is defamatory, unless you provide some evidence.
I think it would be weird and icky, but that is just my prejudices. If it's harmful to the baby, fair enough stop it, but provide me with the evidence that it is harmful first please.
I'm not saying you are necessarily wrong. All I'm saying is I'd like to see the evidence apart from the fact that it makes you feel icky.
There seem to have been various cases in the past of babies having health conditions after ingesting baby formula that has not been properly tested for its harmful effects and which does not have the nutritional value of breast milk. Unfortunately, these things only come to light after the damage has been done to the babies e.g Remedia brand baby formula lacking Vit B1 caused 3 deaths and 20 babies to have severe disabilities.
Many real women apparently feel less of a woman if they can't breast feed their baby - possibly due to cultural pressures on them - not sure how much societies prioritise their need to feel like a woman over what's best for the baby.
As for getting into the mind of a trans person who thinks they are trapped in the 'wrong body' - I don't know what's so special about the mind of a trans person compared to any other person who struggles with their identity. Gender categories are just socially-constructed classifications based on the prevailing culture at the time. If Trans people have developed a sense of identity that doesn't match their body or sex organs, doesn't seem any more difficult to understand than other groups of people who struggle with mental health issues while they try to change what's on the outside to reflect/ match who they feel they are on the inside - clothes, haircuts, wigs, extensive tattoos, body-piercings and surgery/ medical procedures such as fillers, boob jobs, limb-lengthening surgery. Or people who try to change other biological traits to match their sense of identity. Some people try to supress their biological attraction to others or sexual preferences through medical intervention because it does not match their sense of who they are.
Sure, there could also be a biological basis for our sense of identity. For example a study on the Physiological Sensitivity to Disgust as a Predictor of Political Attitudes suggests political attitudes connect to broad, dispositional, perhaps biological temperaments
https://languagelog.ldc.upenn.edu/myl/SmithDisgust2009.pdf.
There could be a biological basis for our sense of gender identity that is different from the basis that classifies our biological sex. But our politics, gender and religious identities still all seem to be social constructs based on meanings we associate with experiences and actions and based on values and beliefs we hold that change as our surrounding culture and experiences changes.
So if I considered myself a Muslim trapped in an atheist's body who felt I needed to mutilate my body to validate my new identity, society might go along with it but it wouldn't be surprising if people objected to me expressing my religious identity by feeding my baby substances that could potentially harm it. It would be even less surprising if people objected to me mutilating my baby to express my religious identity such as having the baby circumcised. If there is some evidence that gender is not mostly a social construct like religion based on meanings we associate with experiences, would like to see some links to that.