Answer:
There is not yet a definitive answer to this question. Without knowing him personally it is impossible to say. But his public statements in interviews have been somewhat ambiguous on the topic of his sexuality. In one interview he clearly describes himself as straight when talking about his experiences in Atlanta as a high-school kid:
"When I went clubbing, the people I had the most fun with were gay men. They were the people that took me on, to the mixed clubs, mixes of gay and straight-those were the most fun in Atlanta. I was the only straight kid back in Tennessee that was really even open to hanging out with gay kids, so that had a lot to do with it." (http://www.thevillager.com/villager_121/thedelldudeshines.html)
In another interview, he again describes himself as straight when discussing the experience of playing a gay character:
"To be exact, my character (Christian) was 'pansexual' and then became fully gay by the end. It was actually very humbling and liberating. Too many guys are worried about 'how they come off' sexually. I just hope that I can be an example that straight men can dance, be sexy, effeminate, and open to the beauty of everyone."(http://www.hunkdujour.com/blog/archives/hunks/ben_curtis/)
However, he hedges on his sexuality a bit when discussing similarities between himself and the Christian character, saying that "[Christian] probably leans more toward the gay side" (http://www.thevillager.com/villager_121/thedelldudeshines.html). Curtis is clearly endorsing the notion of a fluid sexuality here, and indicating that while he is not gay, he may not be completely straight either.
The most forthcoming quote Ben Curtis has made in regards to his own sexuality came when he discussed what it was like when his father came out as gay. Curtis was 12 years old at the time, and he said it was hard because he was trying to figure out his own sexuality at the time, and his father's situation "made it confusing all the way up to this day". Curtis also said playing Christian has made him more comfortable with his own sexuality. The full quote is quite eloquent so here it is in its entirety:
"Yeah, my father is now openly gay. My sister actually came out before my Dad did, but she is really bi-sexual. My father announced to me when my parents separated that he was also bi-sexual but in no way did I doubt his commitment to my mother because I know, just from growing up with him. He's really my best friend and I like him more and more every day I'm alive. He would never be in something he wasn't committed to. They had a beautiful marriage for a while and it fell apart. They're both happier now. It was difficult for me because I was trying to figure out my sexuality then and it made it confusing all the way up to this day. But growing up in Tennessee if you're anything but homophobic when you're a teenager then you're considered gay. Thanks to the passion and sensitivity my parents raised me with, I was able to accept people for who they were and this play has made me more comfortable with my sexuality because I have to deal with it eight times a week. People are really caught up with 'sexual preference' yet I think there are so many other things to worry about."(http://www.broadwayworld.com/article/Hey_Dude_Its_Ben_Curtis_A_visit_with_one_of_the_stars_of_OffBroadways_JOY_20050901)
It is impossible to infer from all this precisely what Ben Curtis' sexuality is. There are some instances where he has categorized himself as straight and others where he has hinted at personal struggles with his own sexuality and moved away from the notion of sexual labels. Regardless, he is clearly an eloquent and insightful young man who was exposed to sexuality issues at an early stage of life, and he appears quite comfortable in accepting himself and accepting others for who and what they are.
Update: Ben Curtis has in fact explicitly answered this question. In an interview from 2005, he described himself as "straight, but not narrow" and answered yes to the question of whether he has fooled around with other guys. He said that he appreciates men just as much as women, but leans more toward the female side. So he self-identifies as straight, but his attractions and experiences could be characterized as bisexual, or alternatively as a three on the seven-point Kinsey scale. Full quote:
Asked if he's ever "experimented" like his character, Christian, Curtis says, "Absolutely, yeah. I appreciate men just as much as women, but I lean toward the female side." So is it fair to say he's straight? "Umm … I'd say I'm straight, but I'm open-minded. As Darryl says in the show, 'I'm straight, but I'm not narrow.'"(http://www.andiamsomebody.com/archives/2005/10/steve_the_dell_guy.php)