It's impulsive and shows Romeo not accepting responsibility for his actions. When Mercutio is killed, Romeo feels guilty that it wasn't him who fought Tybalt. Rather than realising that he did the right thing by ignoring Tybalt's insults, and that Mercutio chose to get involved, Romeo looks for someone else to blame. He could accept responsibility for coming between them, which is how Mercutio was injured. Instead he blames his perceived weakness on Juliet. He has been married barely an hour and already he is angry with Juliet, even though she has done nothing wrong. Ironically, when Juliet hears that Romeo has killed her cousin she in turn blames him, but then rapidly changes her mind.
The quote also reminds us of the pressure in the play to be a strong male. Mercutio and Tybalt both flaunt their masculinity, with threats and crude sexual humour. Look also at the opening scene where the servants want to prove their manhood by making crude jokes and starting a fight. Romeo plays no part in this toxic masculinity until Mercutio is killed. Then he blames Juliet for making him 'effeminate' and for a brief moment acts just like the 'real men'. This is an impulsive act, which Shakespeare uses to bring about Romeo's downfall. It's no coincidence that the only one of the four men present at the fight to survive to the end of the play is Benvolio, who dislikes violence but is much less impulsive.