Understanding and questioning one's sexual attractions can be a very complex and sometimes, confusing time; I appreciate that you've sought out some help to clarify your questions.
By your statement, I understand that you believe you were born straight and have grown up being attracted to women, but you have recently been paying attention to attractive men. You're also curious to know why your orientation changed and what caused it.
First off, it would be helpful to reflect on what you are feeling when you are noticing these attractive males. Do you feel a special emotional bond or attraction? Is the attention mainly focused on sexual feelings? Or perhaps there is another realm of attractiveness you feel Knowing how you feel about men can help you examine your sexuality.
In general, homosexuals are often classified as individuals who are sexually attracted to others of the same sex. At the other end of the spectrum, heterosexuals are those who are attracted to members of the opposite sex. Bisexuals find themselves anywhere in the middle. Note that plenty of heterosexuals can remark about someone of the same sex being attractive without being considered bi- or homosexual.
I will also point out that human sexualities can be very fluid and diverse. Some people consider themselves to be one orientation or the other and carry out relations that contradict their proclaimed sexual orientation. Others opt for a more concrete classification in which they place themselves and never deviate from it. Whichever identity you choose to own is your choice to make; there are no wrong labels to identify yourself as long as you are comfortable with them, and there are plenty to choose from should you feel the need, beyond just bi, gay or straight.
The question of how sexual orientation comes to be is a very tricky one. Yes, most people talk about the theory which hypothesizes that people are born with whatever magical ingredient (so to speak) makes him/her homosexual, bisexual, etc On the other hand, people also subscribe to the theory that one's sexual orientation is entirely up to the environment surrounding them during development. Even still, some postulate that it is a combination of the two that factor in to one's sexual orientation. Additionally, many other theorists have their own explanations on how our sexual orientations come to be. The jury is still out on this issue (and it probably will be for decades to come). This is unfortunate for many individuals, like yourself, who are trying to figure out their sexual orientations. This is due to ever-conflicting findings in research studies that try to reveal more information about human sexuality. If this is any consolationwe're not even sure what factors contribute to heterosexual orientations yet!
In relation to your question about your plentiful friendships with women and its possible impact on your sexual orientation, I would have to say at this point in time that it is not a likely possibility. I will offer that everyone's sexualities come about at different ages and can change over the course of a lifetime. This is how we hear of people discovering their homosexual (or heterosexual) identities well into their 30's, 40's and beyond. There is no right time for sexualities to present themselves; it's completely normal to still be finding oneself later on than some people. I can assure you that you are not the only one who is coming to terms with this idea. There are plenty of archived Q&A's on AlterHeros about this topic. You can read through these questions and see how others your age are going through this process.
With that said, there is one true way to know and understand your sexual feelings, and that is to know yourself and trust your emotions. If you believe that you would be happy in a homosexual relationship, then you are more than able to seek out and experience one.
I hope that this has answered some of your questions and brought about some resolution for you. Please write again if you have more.
Kelley, for AlterHeros