From what my doc explained to me, libido and "functionality" all rest on how much testosterone you have in your system. Over 3 months, I went down from 20.6 nmol/L (which is high in the normal male range) to 0.67 (which is low in the normal female range) and have over the last month lost my ability to sustain an erection hard enough for penetration. According to my doc, this is entirely a consequence of taking oestrogen. Other doctors may see this differently, I don't know.
It certainly seems from reading posts on Susan's that people's experience varies greatly on hormones - some lose no functionality, some lose all of it, and everything in between. So there's no infallible formula to help you be sure you'll keep your hard if you're going to be on HRT. It's probably worth watching your testosterone level, but if it drops too far it takes quite a while to come back up again after if you change your dosages (and in some cases remains permanently low).
I've found it quite difficult to adjust to this myself, as my girlfriend also enjoyed what I had, and we're both going through a letting-go process (before we try out hollow strap-ons). But the effects on libido can be just as significant as the effects on your hardware. My libido has really dropped, and gone 'female' in the sense that I now find getting turned-on to be a matter of conscious decision rather than "oh, there it is". But the sensation is all very good

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This is a mixture of personal experience/info and stuff I've picked up froom others on here, so I hope it's useful but it's not necessarily gospel.
PS: justmeinoz, that's 'her thread', not 'his thread', if you don't mind

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