I need to forward an interface and I wonder why is disable by default? There is any security issue when I enable it? Is there any side effect when I enable it?
ip_forward is only useful one multihomed systems (more than one network interface) and makes routing of network packets between those interfaces possible. By enabling
ip_forward your box essentially becomes a router (gateway).
Routing is an optional network function and as such disabled by default.
As for the downside/possible risks: when one of those interfaces is facing a public subnet (WAN/internet or possible hostile LAN), routing makes attacks possible. That's why you need to address those potential security issues with a firewall.
There is an excellent answer at What is kernel ip forwarding? with more information on the subject.