I completely understand why M/M audiences don't like M/F and can compare it to myself. I write BWWM (black women and white men romance) and that's my favorite genre. I don't read black romance at all and rarely read mainstream (white romance) these days unless it's historical romance. I love historical romance. BWWM is my
preference. I love IR relationships, support them and love white men so I love to read romances with black heroines and white heroes. I don't go out to seek black or mainstream romance for one reason because there is so much BWWM now that I don't have to.
Some think that the black romance readers cross with BWWM or IR readers. Nope. A small percentage but not many. Most who read black romance books won't touch a BWWM book and visa versa.
It's the same with M/M and M/F audiences. Rarely do you see many romance readers who will read any and every pairing. Nope. Most have that ONE favorite pairing and that's what they want to read.
M/M readers will be mighty ticked off if you stick a M/F in the story. Big time. I've seen reviews on M/M books where the readers trashed the author and promised to never read another book from the author again because they felt this was a "betrayal". The same with M/F readers. Many who read M/F don't read M/M but I have seen more of a crossover with M/F readers who are open to M/M than M/M open to M/F. Most the M/M readers I see read ONLY M/M and they want no boobs or vajayjays in their books other than as secondary characters.
It goes back to, we all want what we want. Some readers are more open and others not. For example, if you built a brand on M/M and suddenly started writing M/F and a reader fell in love with you for your M/M, they'd be like, "What the heck is my favorite M/M author doing? Why is she all of a sudden sticking these M/F couples in this series? If I wanted to read M/F I'd read someone else!" And visa versa.
There are some authors who write M/F and M/M but many do them under different pen names and the ones that don't will put a disclaimer on their books. For example, if Becky Lewis is known for M/M and she starts writing M/F or including it in her M/M work, she might make mention of that in the book's description, letting readers know they aren't getting what they would normally expect from Becky. Or she might even send a notice to her mailing list or fan group letting them know, "Hey guys, my next book will include a M/F couple."
Speaking as an author, I am all for you writing the story you want to. But, if you switch stuff up during an existing series, be ready for a possible backlash.
Also, I reread your post. I assumed that all the books in your series were M/M and you included a M/F couple in later on as a side couple or something. I see your FIRST book is M/F and the rest M/M? Don't take this as criticism because I am generally curious, why would you do a M/F book and then make the rest of the books M/M? That's something I've never seen before. I understand you read both, but most do not. Why not just continue the series as M/F or have the first M/M too? You will lose a lot of readers because you will tick off those who enjoyed the first who don't want M/M books and the M/M crowd won't be interested in book 1 because it's M/F. What's gonna hook them into book 1 to even hang around for book 2, etc?
If I were you, I'd break this up and write two different series. Continue the M/F one with M/F only and do a M/M series separately. I'm afraid you will end up getting rejected by both audiences with this type of project.
Also, I disagree with your friend. You don't HAVE to have a series. That's an old indie wives' tale that series are the only thing that can sell. I have three series and the rest of my work is standalones and I have written about 40 or more books. My best sellers have been my standalones. Not everything has to be a series to sell well in self-publishing. That's a myth. There are a lot of authors doing well indie who don't write series. If a book is gonna sell, it's gonna sell standalone or not.
If you don't want to write an M/F series you can make book 1 a standalone. Problem solved on many levels.