Branching paths are definitely better, you can have a story with more focus and form a deeper connection to the central character(s). Plus I personally am not interested in the harem trope, so I'd rather see those deeper, more intimate connections. Though it must be said that for them to be done right requires a lost more development time, so I get why so many games don't do them.
If you are talking about some game that just about the sex, then a single path works just fine, I guess, not like I would be interested in such a game anyway.