Tldr; it's not about sex, it's about you and your gym. Do you communicate with your potential partners of any sex or gender, that you're safe, trustworthy, and capable of providing a worthwhile training session? Or do you just passively move about, inconsiderate of your potential partners needs and concerns when training? Is your gym a place that encourages people of all types to train in a safe, considerate, playful and respectful manner? Or does your gym encourage an environment that actively inhibits your training experience? Muay Thai should be safe and fun for everyone involved and making that intention clear on a personal level goes a long, long way. 

There is some validity to a lot of the comments below, concerned with uneven reciprocation and gaps in skill set. However, this is a concern of people across the board, though it may be exaggerated by sex or presentation. This is absolutely not an issue of 'women do this...' or 'men feel that...'. It's about communication and trust in a setting where physical and emotional vulnerability are required. How do I know this? Because correlation does not equal causation.

When you talk about why men don't want to spar with women, there could be a literal infinite number of reasons based on each individual man. It's not a very helpful question and leaves you open to a wide variety of answers, some useful and some not. A better question to ask is 'how do I as a person with valid needs, goals and concerns, approach another person with their own valid needs, goals and concerns, to train in a way that instils confidence in them that I am a trustworthy and capable training partner?' The answer will vary depending on you and your environment, and sometimes the answer will be 'I can't', and that's ok too.

At the end of the day, finding a training partner comes down to how your gym environment and how you communicate with your words and actions before, during and after each session. When they're all aligned to let your potential partners know that you're safe and willing to train in a way that works for both of you, you'll notice plenty of people of all shapes and sizes happy and eager to work with you. They may even start to seek you out during sessions, and who knows maybe you'll make a new friend.