charlie horse durring sex!?

Chances are these are not really muscle pains but are signals from nerve pressure elsewhere in the body, possibly the spine...

Poor posture, poor abdominal muscle tone, spinal problems, etc. can cause pressure or even damage to nerves often resulting in various forms of leg pain, the most common form is sciatica - pressure on the sciatic nerve in the spinal column gives an often debilitating pain down the outside of the leg. Often sciatic pain can take a deal of time to disappear...

I suffer from similar "muscle" pain down the front of (mostly) both thighs which sometimes even make my knees ache...
In spite of the attentions of a physiotherapist I find the problem hasn't reduced and I get instant relief by giving myself a little traction - taking my weight on my arms by pushing down between convenient items of furniture for a minute or two.

Some people find reaching up and hanging from a door frame or ladder works best for them but for me it makes things worse...

Could be time you visited the doc or an osteopath?