It's a matter of opinion, but there are some talented women that have all shaped music today such as Whitney Houston, Adele, Dido, Amy Winehouse, Tina Turner, Shirley Bassey, Diana Ross, Kate Bush, Arethra Franklin, Cyndi Lauper, Cher, Sarah McLachlan, Tracy Chapman, Julie Andrews, Alicia Keys, Judy Garland, Sarah Brightman and Katherine Jenkins (for opera-style music)
Bands including The Ronettes, The Pointer Sisters, Spice Girls (come on, they were very successful!) Destiny's Child, The Supremes (with Diana Ross) ABBA (male and female)
It's definitely a matter of opinion in a lot of cases. As for music back in Beethoven's day, well I don't think it was particularly normal for a woman to get into the music business so they'd have been less likely to spread any talent that they might have had. I'll admit that men tend to dominate when it comes to music legends (only just), but it can't be because women haven't the capacity because there are women who have done. Like I said though, it's all opinion. You're also only really thinking in terms of rock style music.
edit: Oh I knew I shouldn't have mentioned the Spice Girls... but they made my childhood!