I've never really had a problem with it, though it depends on the people involved too. The major concerns for me with being sexually active with more than one partner are health-based: there's obviously more care to be taken in terms of safe sex if you're having multiple partners. The risk of STIs is something you especially have to think about if each of the partners you have also has other partners.

In terms of time and energy expended, I'd think that would depend on how serious each of the relationships were. I've been in situations before where I had one partner that shared one interest with me and another who shared another, so there was a lot of "blah night I have the gaming date" and "blah other night I'm staying in with other lover to watch movies". It really wasn't a problem for anyone involved, honestly.

When relationships do get serious, that becomes a bit more of an issue, and then you have to get into some major polyamory politics, perhaps. But having more than one casual relationship has worked out fine for me in the past.

(That is, until I got busy with parenthood, and now I can't even manage one relationship oops.)