Yes Eunuchs can sustain and erection, seriously, google it.
This however has nothing to do with Lord Bealish. She is being fucked by Strong Belwas who is Queen Daenerys Targaryen's bodyguard in the books. This was just meant to be a quick scene before she meets and fucks the Queen. But I never got around to making the Daenery's scene. Maybe soon.
Season 4, episode 3, "Breaker of Chains." It was a scene that also took place in the "A Storm of Swords" novel, except there was defecation instead of urination, and a different character performed the act.
In the novels, it's a character named Strong Belwas -- a massive eunuch formerly of the fighting pits of Meereen who allies himself with Daenerys Targaryen -- who is made Dany's champion in her show of force against Meereen. After winning the fight, he relieves his bowels as an insult to the rulers of Meereen. Since Strong Belwas was cut from the TV series, it was decided that Daario would get to perform a slightly tweaked version of Belwas' most epic scene instead.
Pity he didn't make the TV series but at least now I won't have people saying 'that doesn't look like Strong Belwas'.