Not only are the buildings heated, but it *IS* a leather event... leather is warm. It wouldn't make sense to hold a leather event in DC in, say, August.
As Rawtop posted, some guys go just for the sex (which is fine, everyone has his own motivations). But I'd add a few other observations:
1. IML being international, brings in a much more geographically diverse crowd. For many it's a "stand and model" beauty pageant both at the contest and at many events, and there can be (among some participants) a lot of body snobbery. It's not universal but be forewarned, if you can't deal with rejection it's not necessarily the best place to be.
2. MAL is smaller, more locally focused (at least, primarily east coast up to about the near midwest), and is much more of a traditional leather event. Unlike IML, MAL started as a social event put on by a leather club, and only later added the contest as a feeder for IML. So a far higher percentage of attendees are into leather and BDSM, although conversely IML, because of its appeal worldwide, attracts some of the most hard-core players. At MAL in particular, If you aren't in serious leather (i.e. considerably more than an armband on your right bicep), you may well be ignored at most of the social events.
In other words, these are not sex parties although sex parties may well be individually held events at both venues. Don't go assuming you pay your admission fee and automatically get to hop in some sling in a big dungeon room somewhere.
BootmanLA