One track mind means that you are only thinking about one thing, one topic. Native english speakers would interpret it as 'only thinking about sex'. It does not necessarily mean you are stupid. Intelligent people often have a 'one track mind'.

It means that Sally is only thinking about one thing. For example, if all I ever talk about or think about is girls, someone could tell me that I have a one track mind. It is typically used to mean that someone is stupid, but I think it means also that they are ignorant. Does that explanation help?

On a race track, the runner must go in a particular direction until the race is over; someone with a one track mind is like the runner, in that they pursue one goal without distraction. A person with a one track mind for sex is always only thinking about sex. The person would have no other interests that would come before his or her desire for sex. Anything the person says or does would likely be in pursuit of sex.

In American English, "one-track mind" tends to mean one is enthusiastic about one thing to the exclusion of others. It's another way of saying that someone is "focused." This isn't necessarily negative, but it may express displeasure on the part of someone who is ignored by the focused person more than anything else. Definitely does not mean "stupid."