How to explain homosexuality to a young teenager?

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My 13 year old grandson called me and confided he caught his 14 year old sister and her same-age female classmate/friend "experimenting" in her bedroom with her door ajar. When I asked for details, he was a little shy at first, then described they were kissing and touching each other but were fully clothed. Apparently my daughter was not home from work yet and my granddaughter didnt expect not to hear her brother come home from his school. I asked him why he didnt ask his mother and he said he didnt want her in "trouble". We are an open-minded family and the children were raised knowing the different aspects of sexualities and how not to condemn others' right to choice unless they intend to harm others or themselves. However, this hits close to home and explaining her rightful choice to experiment was difficult for him, who had girls calling him day and night, to swallow. I did tell him to tell his mother he spoke with me and inform her of his accidental discovery. I believe my granddaughter had already done so anyway by that time. When I was 15, my favorite male cousin came out of the closet at a family reunion by pecking his guest from college on the cheek after they won a round of horseshoes with the male relatives. That one and only peck caused a lot of anxirty I never forgot but appreciated to this day. But I didnt have anyone to talk to back in the 60's - you didnt ask 'those' questions! How would you handle it if your son, daughter or grandchild came to you about witnessing a relative's obvious venture into homosexuality?