Because they are still ignorant and/or misinformed about what sexual orientations are and what it means to be homosexual. Very often, those who are bigoted against gay people think homosexuality has only to do with actions, ie. actual sexual activities between two people of the same sex, and since actions are a choice they think homosexuality is also a choice.
A person can choose whether or not to engage in any particular sexual activity, in this case sexual activity with another person of the same sex There are gay people who are in a lasting relationship with another person of the same sex, and they never do anything sexual together whether due to lack of desire/sex drive or lack of ability (I'm not including kissing in that because kissing is not an inherently sexual activity). For those who do have a sex drive and the ability to have sex, they can choose whether or not to engage in that activity. But their core feelings of attraction for the same sex are never a choice. It's hard-wired into the brain from before they were born.
As far as people saying it's a "phase", that also comes from ignorance about it. They just think you're temporarily into something taboo, and in time it will pass as you lose interest. They have no idea, or refuse to believe, that it's something there to stay because you can't change how your brain works any more than they can change the way their brain works.