It is not a sin of you think of sexuality as long as you did no action. The prophet (saw) said:
“The Prophet (ﷺ) narrated about his Lord I’m and said, “Allah ordered (the appointed angels over you) that the good and the bad deeds be written, and He then showed (the way) how (to write). If somebody intends to do a good deed and he does not do it, then Allah will write for him a full good deed (in his account with Him); and if he intends to do a good deed and actually did it, then Allah will write for him (in his account) with Him (its reward equal) from ten to seven hundred times to many more times: and if somebody intended to do a bad deed and he does not do it, then Allah will write a full good deed (in his account) with Him, and if he intended to do it (a bad deed) and actually did it, then Allah will write one bad deed (in his account) .””