It’s 2019. Support the industry.
Here is an (incomplete) list. It will be updated gradually.
Official Releases:
Bookwalker: https://global.bookwalker.jp/genre/15/
June Manga: https://junemanga.com/
Renta: https://www.ebookrenta.com/renta/
SuBLime: https://www.sublimemanga.com/
Free BL:
Lezhin: https://www.lezhin.com/en
You can read a handful of premium BL series under their Wait Until Free line: https://www.lezhin.com/en/page/wuf_181114_1
Manga.Club: https://www.manga.club/book/genre/Yaoi+Manga/
Some titles require tickets, however, they have multiple free options.