For those of you have read the book and seen the show, why did they make Renly gay in the show? i've only read the first book but does this come out in the second book or something?

Both Renly & Loras are gay in books, George has confirmed link, but it is never been directly said in the books. But it is hinted several times.

Few examples:

Now sheathe your bloody sword, or I’ll take it from you and shove it up some place even Renly never found. Jaime to Loras

When the sun has set, no candle can replace it. Loras about Renly

Renly’s little rose? I doubt that. Oberyn about Loras

We had...we had prayed together that night. Loras about Renly

...I got Margaery. You’ll be pleased to know she came to me a maid.
In your bed she’s like to die that way. Renly & Stannis

And it relieved him {Mace} of the difficult task of trying to find lands and a bride for a third son, never easy, and doubly difficult in Ser Loras’s case. Petyr

Loras is valiant and handsome and we all love him dearly...but your Imp will make a better husband. Garlan

Renly also has a reputation for being well into fashion and the best dressed man at court

And instead of a standard Kingsguard, Renly decides to call his group the “Rainbow Guard.”