It's WAY easier for a 15 year old girl to be pressured into sex by an adult man for many reasons.
For starters, it's typically seen as "more cool" for a younger girl to date an older guy. A young teenage girl can think she's impressing her friends and this older guy by having sex with him.
Older guys typically have more money, which can impress many younger girls. Most guys their own age are making $6/hour at McDonalds, whereas men are making "real" money.
Adults tend to live on their own... and although it does only take a few minutes to have sex and it can be done anywhere, it's easier to be pressured into something when you're alone in someone's house.
Men typically have more sexual experience than, say, a 15 or 16 year old boy. If a 16 year old girl has sex with a 16 year old boy, generally speaking, neither one knows what they're doing
For the most part, sex isn't as big of a deal to a 25 year old guy.
I wasn't a teenager too long ago, and I've always considered myself to be mature for my age. I've seen my friends get pressured into having sex with guys they were barely dating/didn't really like or know/felt they had to.
Of course, this is all my opinion. I know it's the less popular one here, but I'm sticking to it
My best friend in high school lost her virginity at age 17 to a man who was almost 30. He went to her church and was their youth leader. He was "friends" with the entire youth group... everyone thought he was a really cool guy. Anyway, my friend had sex with him in a parking garage in his car. She really regretted it afterwards, but felt like she had to have sex with him because he was older, in a position at her church, and everyone thought he was a really cool guy.
Age is a WAY bigger deal when you're younger. A 16 year old and a 20 year old should be in completely different parts of their lives! When I was 16 I was just getting my driver's license. When I was 20 I was a Junior in college. It's not a big deal for a 25 year old to go out with a 35 year old, a 30 year old to go out with a 45 year old, etc, etc... because they are both adults and in closer life stages.