As a straight male, I’m not going to be swallowing semen; so maybe this is a double-standard. But why do so many women not want to swallow? My ex-fiancée thought it was unhygenic. Others simply said no. Receiving fellatio, but knowing it won’t be to completion is a bit of a turn-off. The alternatives are to use a condom, or to make a bit of a mess.
Slightly related: Do you mind if the guy makes suggestions about how to fellate better?