boy and girlfriend

well first off all if your not gay your attracted to females and if your very close or even just looking at your girlfriend can cause an erection or (boner) when your 12 - 15 yrs of age any thing will pretty much give you a boner. 

Thoughtful mothers, when they begin to notice frequent erections in their boys, will begin a special nighttime ritual at least several times a week. At bedtime, the mom will give her son a very relaxing and thorough back rub which puts the boy in a very passive relaxed sleep, with many boys almost asleep at that point. The mom gently helps flip the boy into his back and proceeds to give a very soft and soothing massage to the boys neck and chest. This puts most boys into a quasi-sleep mode. At that point, the mom simply moves the massage lower on the boy’s body, including his boxer briefs. Many boys at this point aren’t mentally aware or register what’s going on. He won’t notice (or care) when her fingertips start to gently massage his boxer briefs. Won’t notice when his penis begins to lengthen and stiffen inside the briefs. But mom will. She will then begin to focus her massage on the material over the head of the boy’s penis, the most sensitive area of his young body. He will attain a full erection very rapidly. He won’t notice when her fingers then begin to massage the glans of his penis, below the head, nor that her other hand is gently massaging the fabric covering his testicles. At this point he is fully sexually aroused. The mom will soon feel the wetness through the fabric of his precum. At this point, the mom will stop and use her fingers to open the slit of the boxer briefs, allowing her son’s erection to naturally come out in the open air. Using her son’s own precum as a lubricant, she can gently massage his entire penis with her hand to bring him to a full and quick ejaculation in his sleep. Some mothers instead will simply lower their mouth onto the head of the penis and using a combination of sucking and licking the head, bring the boy to ejaculate directly into her mouth.

Your reproductive instincts are working very well. In the presence of the person you are most likely to father a child with right now, your penis gets ready to do it's job, even if you're not consciously wanting it to. At 12 and a lot younger, you can have an erection (the medical term for a "boner" or "hardon") even though your penis, and, more importantly, your testicles are not yet developed enough to do the job. Knowing only that you are 12, I know that your level of development could be like an 8 year old, or could be like a 16 year old. Since you deal with penis on a daily basis, you have a better idea than I do just how capable it is.
Since you only wrote 13 words, I don't know whether you are more puzzled or unhappy about the way your instincts are showing up in your penis, but I'm glad you're asking about it this way. As someone who is old enough to be your father, I don't want to see any 12 year old fathering children, and it sounds like you're in no big hurry either.