Heres the deal, for me personally I do not like my breasts, I do not like to be groped, its not a feature of myself I like. I do like nipple play, however, thats gender neutral.
Keep in mind your partner IS a man, not a woman with masculine characteristics. Theyre going to get rid of their breasts someday, they may even pass as a man someday, will you still like them the same then? Or are you limited to their body type?
On my end Ive had similar questions Ive asked myself, Ive always been attracted to males, feminine males even. One day my partner decided she was trans. Was this going to be a turn off? I had to fight with myself...'well, as long as she kept her penis' 'well...as long as she had small breasts'. No, no, no. In the end, if you cant love them for who they are, not who you want them to be, its not a real relationship. I decided in the end that I did not love them for their penis, but for them, and supported them in their transition. They were a woman to me and not a man.
Always remember, your friend is a man. Ask them what bithers their dysphoria. Groping of breasts could be one, even having a vagina may be weird. Do not love this person for something they are not.
Odds are though they might let you have full on intercourse regardless of dysphoria. As much as I dislike the lower area I use it quite often, simply because its all I have and it gives me pleasure, I cannot do without it in sexual encounters, my quirk is I hate getting looked at in lighted areas, its uncomfortable.