I enjoy reading BL. Not really into Shoujo, but right now I'm watching a Shoujo anime in Netflix. And I'm really enjoying it.
I just got the idea. What if there's a Shoujo manga that has a subplot that's Bl in nature?
Is there anything like it?

I guess Watashi ga Motete Dousu n da? I've never read it before but based on the tags and description you might like it?

________________
I love gender bender and delinquent manga

I don't know exactly what you're looking for regarding the main CP because, when it comes to shoujo + shounen-ai as a sub-plot, most of the titles that belong to that classification have a male/male CP rather than a male/female.

However, there are a few exceptions.

Something that mixes concepts in both the same main CP and side CPs is UraBoku. It deals with reincarnation, so the main couple is originally male/female, but in this life, they're males. The same goes for other side pairings.

Another title that comes to my mind is Tsubasa: Reservoir Chronicle. Main CP is Syaoran/Sakura. And there, you also find the typical allegory to shounen-ai CLAMP uses to add as a spicy element in Kurogane and Fei. I wouldn't recommend this title at all, though. It kind of bored me most part of the time.

Hana wa Nisemono and Daites Ryou Koubouki might be of your interest as well.
The latter follows, more or less, the line of the title tylurr5 suggested above.

Now, if you're looking for something "shoujo-ish" (or josei-ish) and BLish, take a look at some of Margaret's and Romiko comics' titles.

Nyotaika BL is a BL sub-genre you might want to check out. It's technically gender bender, but in these titles, the uke transforms into a woman because of genetics, curse, etc. One example is Ichinose-kun Wa Kofun Dekinai. As you can see, there you have both smutty straight and BL romance in just one story.

________________

ᵂᵃʳᵘᶦ ᴷᵒᵗᵒ ˢʰᶦᵗᵃᶦ ᴳᵘᶦᵈᵉ | ᴳᵉⁿᵉʳᵃˡ ᶦⁿᶠᵒ | ᵂᵃʳᵘᶦⁿᵉˢˢ