Allah's fitrah on intimate relationships between humans
Allah explained His fitrah on the matter in several verses:
Both verses actually say that the pure nature (fitrah) which was given all humans at their birth is that of having a mate of the opposite gender (later). And which leads to off-spring.
Beside this all rulings related to a an intimate relationship in the Qur'an addresses the relations between partners/mates of both or more precisely opposite genders.
Some evidences for homosexuality being against the fitrah according the Qur'an
And Allah () in the Qur'an clearly condemns homosexuality or the doing of the people of Luth -which actually leads to the conclusion that it must be against the fitrah:
Here the Luth actually point at the mismatch between Allah's fitrah and the practice of the people of Sodom.
And [mention] Lot, when he said to his people, "Do you commit immorality while you are seeing? (54)
Do you indeed approach men with desire instead of women? Rather, you are a people behaving ignorantly." (27:54-55)
Here the Qur'an calls it immorality, and we know that Allah doesn't forbids immorality so how would he make it a part of the pure nature (fitrah) (see for example 16:90) in opposition to what Satan orders (see for example 24:21).
And [mention] Lot, when he said to his people, "Indeed, you commit such immorality as no one has preceded you with from among the worlds. (28)
Indeed, you approach men and obstruct the road and commit in your meetings [every] evil." And the answer of his people was not but they said, "Bring us the punishment of Allah , if you should be of the truthful." (29:28-29)
The first of the quoted verses again states that the people of Luth were the innovators or initiators of this immorality, while the second one points at the mismatch with the fitrah.
Also note that Luth tried to convince them by even offering his own daughters for marriage as stated in several verses.
So a part of the kufr (disbelieve) of the people of Luth was their rejection to live according Allahs rules and his fitrah.