Radical,
What is your definition of "sexual undertone"? If "sexual undertone"=thinking of having sex, while "no sexual undertone"= compeletely neutral relationship, then definitely no.
I have a much more nuanced approach to this. There are a lot of degrees and of different ways between these two extremes. Some people give you a feeling of physical closeness (you'll want to hug them or give them a friendly kiss), with some people you feel admiration, with some people you feel a great complicity (you feel right away like you've known each other forever). None of these are neutral relationships, but they are not really sexual either. It's more like different levels of sensuality. It is hard to notice it or accept it even, because we are so conditionned to consider relationships on a 100% intellectual level, or intellectual+sex.
By the way, I don't think the gay/straight distinction is that binary. A friend of mine who is bi told me about a study (sorry I can't remember the source) stating that only a very small percentage of people are 100% gay, or 100% straight, or 100% bi (feel exaclty the same attraction for either gender)
For most people, our interactions are on a more subtle scale than this. Whether people are aware of it or not, whether people accept it or not. Can't you think of friendships in which there was more than intellectual bonding?
I noticed guys (more than girls) have a lot of trouble accepting this idea, they'll be like "are you calling me gay???"
I'm not

I'm saying that perfectly neutral relationships, with people from any gender, are very rare, whatever your sexual inclination is. This is why, even without any sexual afterthought, hugs, kisses and cuddles feel so good.